Cloudflare Docs
Edit this page
Report an issue with this page
Log into the Cloudflare dashboard
View GitHub RSS feed
Set theme to dark (⇧+D)

Compatibility dates

Cloudflare regularly updates the Workers runtime. These updates apply to all Workers globally and should never cause a Worker that is already deployed to stop functioning. Sometimes, though, some changes may be backwards-incompatible. In particular, there might be bugs in the runtime API that existing Workers may inadvertently depend upon. Cloudflare implements bug fixes that new Workers can opt into while existing Workers will continue to see the buggy behavior to prevent breaking deployed Workers.

The compatibility date (and flags) are how you, as a developer, opt into these runtime changes. Compatibility flags will often have a date in which they are enabled by default, and so, by specifying a compatibility_date for your Worker, you can quickly enable all of these various compatibility flags up to, and including, that date.

​​ Setting compatibility date

When you start your project, you should always set compatibility_date to the current date. You should occasionally update the compatibility_date field. When updating, you should refer to this page to find out what has changed, and you should be careful to test your Worker to see if the changes affect you, updating your code as necessary. The new compatibility date takes effect when you next run the npx wrangler deploy command.

There is no need to update your compatibility_date if you do not want to. The Workers runtime will support old compatibility dates forever. If, for some reason, Cloudflare finds it is necessary to make a change that will break live Workers, Cloudflare will actively contact affected developers. That said, Cloudflare aims to avoid this if at all possible.

However, even though you do not need to update the compatibility_date field, it is a good practice to do so for two reasons:

  1. Sometimes, new features can only be made available to Workers that have a current compatibility_date. To access the latest features, you need to stay up-to-date.
  2. Generally, other than this page, the Workers documentation may only describe the current compatibility_date, omitting information about historical behavior. If your Worker uses an old compatibility_date, you will need to continuously refer to this page in order to check if any of the APIs you are using have changed.

​​ Via Wrangler

The compatibility date can be set in a Worker’s wrangler.toml file.

# Opt into backwards-incompatible changes through April 5, 2022.
compatibility_date = "2022-04-05"

​​ Via the Cloudflare Dashboard

When a Worker is created through the Cloudflare Dashboard, the compatibility date is automatically set to the current date.

The compatibility date cannot be updated on the Cloudflare Dashboard at this time.

​​ Via the Cloudflare API

The compatibility date can be set when uploading a Worker using the Workers Script API or Workers Versions API in the request body’s metadata field.

If a compatibility date is not specified on upload via the API, it defaults to the oldest compatibility date, before any flags took effect (2021-11-02). When creating new Workers, it is highly recommended to set the compatibility date to the current date when uploading via the API.

​​ Setting compatibility flags

In addition to setting a compatibility_date, you may also provide a list of compatibility_flags, which enable or disable specific changes.

Most developers will not need to use compatibility_flags. compatibility_flags can be useful if you want to help the Workers team test upcoming changes that are not yet enabled by default, or if you need to hold back a change that your code depends on but still want to apply other compatibility changes.

​​ Via Wrangler

Compatability flags can be set in a Worker’s wrangler.toml file.

This example enables the specific flag formdata_parser_supports_files, which is described below. As of the specified date, 2021-09-14, this particular flag was not yet enabled by default, but, by specifying it in compatibility_flags, we can enable it anyway. compatibility_flags can also be used to disable changes that became the default in the past.

# Opt into backwards-incompatible changes through September 14, 2021.
compatibility_date = "2021-09-14"
# Also opt into an upcoming fix to the FormData API.
compatibility_flags = [ "formdata_parser_supports_files" ]

​​ Via the Cloudflare Dashboard

Compatibility flags cannot be set or updated on the Cloudflare Dashboard at this time.

​​ Via the Cloudflare API

Compatibility flags can be set when uploading a Worker using the Workers Script API or Workers Versions API in the request body’s metadata field.

​​ Compatibility flags

​​ Node.js compatibility flag

A growing subset of Node.js APIs are available directly as Runtime APIs, with no need to add polyfills to your own code. To enable these APIs in your Worker, add the nodejs_compat compatibility flag to your wrangler.toml:

compatibility_flags = [ "nodejs_compat" ]

As additional Node.js APIs are added, they will be made available under the nodejs_compat compatibility flag. Unlike most other compatibility flags, we do not expect the nodejs_compat to become active by default at a future date.

The Node.js AsyncLocalStorage API is a particularly useful feature for Workers. To enable only the AsyncLocalStorage API, use the nodejs_als compatibility flag.

compatibility_flags = [ "nodejs_als" ]

​​ Change history

Newest changes are listed first.

​​ Properly extract blob MIME type from content-type headers

Default as of2024-06-03
Flag to enableblob_standard_mime_type
Flag to disableblob_legacy_mime_type
When calling response.blob.type(), the MIME type will now be properly extracted from content-type headers, per the WHATWG spec.

​​ Selecting the correct mime type for a Blob from a fetch

Default as of2024-06-03
Flag to enableblob_standard_mime_type
Flag to disableblob_legacy_mime_type

In the original implementation of the fetch Body mixin, the runtime would incorrectly assign the value of the Blob’s MIME type property when the Content-Type header provided multiple or invalid values. This behavior is inconsistent with the fetch() standard which defines a specific algorithm for determining the MIME type of a Blob.

When the blob_standard_mime_type flag is used, the fetch Body mixin will implement the correct standard behavior.

​​ Use standard URL parsing in fetch()

Default as of2024-06-03
Flag to enablefetch_standard_url
Flag to disablefetch_legacy_url
The fetch_standard_url flag makes fetch() use WHATWG URL Standard parsing rules. The original implementation would throw TypeError: Fetch API cannot load errors with some URLs where standard parsing does not, for instance with the inclusion of whitespace before the URL. URL errors will now be thrown immediately upon calling new Request() with an improper URL. Previously, URL errors were thrown only once fetch() was called.

​​ Returning empty Uint8Array on final BYOB read

Default as of2024-05-13
Flag to enableinternal_stream_byob_return_view
Flag to disableinternal_stream_byob_return_undefined

In the original implementation of BYOB (“Bring your own buffer”) ReadableStreams, the read() method would return undefined when the stream was closed and there was no more data to read. This behavior was inconsistent with the standard ReadableStream behavior, which returns an empty Uint8Array when the stream is closed.

When the internal_stream_byob_return_view flag is used, the BYOB read() will implement standard behavior.

const resp = await fetch('');
const reader = resp.body.getReader({ mode: 'byob' });
await result = await Uint8Array(10));
if (result.done) {
// The result gives us an empty Uint8Array...
console.log(result.value.byteLength); // 0
// However, it is backed by the same underlying memory that was passed
// into the read call.
console.log(result.value.buffer.byteLength); // 10

​​ Brotli Content-Encoding support

Default as of2024-04-29
Flag to enablebrotli_content_encoding
Flag to disableno_brotli_content_encoding
When the brotli_content_encoding compatibility flag is enabled, Workers supports the br content encoding and can request and respond with data encoded using the Brotli compression algorithm. This reduces the amount of data that needs to be fetched and can be used to pass through the original compressed data to the client. See the Fetch API documentation for details.

​​ Durable Object stubs and Service Bindings support RPC

Default as of2024-04-03
Flag to enablerpc
Flag to disableno_rpc

With this flag on, Durable Object stubs and Service Bindings support RPC. This means that these objects now appear as if they define every possible method name. Calling any method name sends an RPC to the remote Durable Object or Worker service.

For most applications, this change will have no impact unless you use it. However, it’s possible some existing code will be impacted if it explicitly checks for the existence of method names that were previously not defined on these types. For example, we’ve seen code in the wild which iterates over bindings and tries to auto-detect their types based on what methods they implement. Such code will now see service bindings as implementing every method, so may misinterpret service bindings as being some other type. In the cases we’ve seen, the impact was benign (nothing actually broke), but out of caution we are guarding this change behind a flag.

​​ Handling custom thenables

Default as of2024-04-01
Flag to enableunwrap_custom_thenables
Flag to disableno_unwrap_custom_thenables

With the unwrap_custom_thenables flag set, various Workers APIs that accept promises will also correctly handle custom thenables (objects with a then method) that are not native promises, but are intended to be treated as such). For example, the waitUntil method of the ExecutionContext object will correctly handle custom thenables, allowing them to be used in place of native promises.

async fetch(req, env, ctx) {
ctx.waitUntil({ then(res) {
// Resolve the thenable after 1 second
setTimeout(res, 1000);
} });
// ...

​​ Fetchers no longer have get/put/delete helper methods

Default as of2024-03-26
Flag to enablefetcher_no_get_put_delete
Flag to disablefetcher_has_get_put_delete

Durable Object stubs and Service Bindings both implement a fetch() method which behaves similarly to the global fetch() method, but requests are instead sent to the destination represented by the object, rather than being routed based on the URL.

Historically, API objects that had such a fetch() method also had methods get(), put(), and delete(). These methods were thin wrappers around fetch() which would perform the corresponding HTTP method and automatically handle writing/reading the request/response bodies as needed.

These methods were a very early idea from many years ago, but were never actually documented, and therefore rarely (if ever) used. Enabling the fetcher_no_get_put_delete, or setting a compatibility date on or after 2024-03-26 disables these methods for your Worker.

This change paves a future path for you to be able to define your own custom methods using these names. Without this change, you would be unable to define your own get, put, and delete methods, since they would conflict with these built-in helper methods.

​​ Queues send messages in JSON format

Default as of2024-03-18
Flag to enablequeues_json_messages
Flag to disableno_queues_json_messages
With the queues_json_messages flag set, Queue bindings will serialize values passed to send() or sendBatch() into JSON format by default (when no specific contentType is provided).

​​ Suppress global importScripts()

Default as of2024-03-04
Flag to enableno_global_importscripts
Flag to disableglobal_importscripts
Suppresses the global importScripts() function. This method was included in the Workers global scope but was marked explicitly as non-implemented. However, the presence of the function could cause issues with some libraries. This compatibility flag removes the function from the global scope.

​​ Node.js AsyncLocalStorage

Flag to enablenodejs_als
Flag to disableno_nodejs_als
Enables the availability of the Node.js AsyncLocalStorage API in Workers. This API allows you to store data that is accessible to all asynchronous operations within a given execution context. This is useful for storing data that is relevant to the current request, such as request-specific metadata or tracing information.

​​ Python Workers

Default as of2024-01-29
Flag to enablepython_workers
This flag enables first class support for Python. Python Workers implement the majority of Python’s standard library, support all bindings, environment variable, and secrets, and integration with JavaScript objects and functions via a foreign function interface.

​​ WebCrypto preserve publicExponent field

Default as of2023-12-01
Flag to enablecrypto_preserve_public_exponent
Flag to disableno_crypto_preserve_public_exponent
In the WebCrypto API, the publicExponent field of the algorithm of RSA keys would previously be an ArrayBuffer. Using this flag, publicExponent is a Uint8Array as mandated by the specification.

​​ Vectorize query with metadata optionally returned

Default as of2023-11-08
Flag to enablevectorize_query_metadata_optional
Flag to disablevectorize_query_original
A set value on vectorize_query_metadata_optional indicates that the Vectorize query operation should accept newer arguments with returnValues and returnMetadata specified discretely over the older argument returnVectors. This also changes the return format. If the vector values have been indicated for return, the return value is now a flattened vector object with score attached where it previously contained a nested vector object.

​​ WebSocket Compression

Default as of2023-08-15
Flag to enableweb_socket_compression
Flag to disableno_web_socket_compression

The Workers runtime did not support WebSocket compression when the initial WebSocket implementation was released. Historically, the runtime has stripped or ignored the Sec-WebSocket-Extensions header – but is now capable of fully complying with the WebSocket Compression RFC. Since many clients are likely sending Sec-WebSocket-Extensions: permessage-deflate to their Workers today (new WebSocket(url) automatically sets this in browsers), we have decided to maintain prior behavior if this flag is absent.

If the flag is present, the Workers runtime is capable of using WebSocket Compression on both inbound and outbound WebSocket connections.

Like browsers, calling new WebSocket(url) in a Worker will automatically set the Sec-WebSocket-Extensions: permessage-deflate header. If you are using the non-standard fetch() API to obtain a WebSocket, you can include the Sec-WebSocket-Extensions header with value permessage-deflate and include any of the compression parameters defined in RFC-7692.

​​ Strict crypto error checking

Default as of2023-08-01
Flag to enablestrict_crypto_checks
Flag to disableno_strict_crypto_checks

Perform additional error checking in the Web Crypto API to conform with the specification and reject possibly unsafe key parameters:

  • For RSA key generation, key sizes are required to be multiples of 128 bits as boringssl may otherwise truncate the key.
  • The size of imported RSA keys must be at least 256 bits and at most 16384 bits, as with newly generated keys.
  • The public exponent for imported RSA keys is restricted to the commonly used values [3, 17, 37, 65537].
  • In conformance with the specification, an error will be thrown when trying to import a public ECDH key with non-empty usages.

​​ Strict compression error checking

Default as of2023-08-01
Flag to enablestrict_compression_checks
Flag to disableno_strict_compression_checks
Perform additional error checking in the Compression Streams API and throw an error if a DecompressionStream has trailing data or gets closed before the full compressed data has been provided.

​​ Bot Management data

Default as of2023-08-01
Flag to enableno_cf_botmanagement_default
Flag to disablecf_botmanagement_default

This flag streamlines Workers requests by reducing unnecessary properties in the object.

With the flag enabled - either by default after 2023-08-01 or by setting the no_cf_botmanagement_default flag - Cloudflare will only include the Bot Management object in a Worker’s if the account has access to Bot Management.

With the flag disabled, Cloudflare will include a default Bot Management object, regardless of whether the account is entitled to Bot Management.

​​ URLSearchParams delete() and has() value argument

Default as of2023-07-01
Flag to enableurlsearchparams_delete_has_value_arg
Flag to disableno_urlsearchparams_delete_has_value_arg

The WHATWG introduced additional optional arguments to the URLSearchParams object delete() and has() methods that allow for more precise control over the removal of query parameters. Because the arguments are optional and change the behavior of the methods when present there is a risk of breaking existing code. If your compatibility date is set to July 1, 2023 or after, this compatibility flag will be enabled by default.

For an example of how this change could break existing code, consider code that uses the Array forEach() method to iterate through a number of parameters to delete:

const usp = new URLSearchParams();
// ...
['abc', 'xyz'].forEach(usp.delete.bind(usp));

The forEach() automatically passes multiple parameters to the function that is passed in. Prior to the addition of the new standard parameters, these extra arguments would have been ignored.

Now, however, the additional arguments have meaning and change the behavior of the function. With this flag, the example above would need to be changed to:

const usp = new URLSearchParams();
// ...
['abc', 'xyz'].forEach((key) => usp.delete(key));

​​ Use a spec compliant URL implementation in redirects

Default as of2023-03-14
Flag to enableresponse_redirect_url_standard
Flag to disableresponse_redirect_url_original
Change the URL implementation used in Response.redirect() to be spec-compliant (WHATWG URL Standard).

​​ Dynamic Dispatch Exception Propagation

Default as of2023-03-01
Flag to enabledynamic_dispatch_tunnel_exceptions
Flag to disabledynamic_dispatch_treat_exceptions_as_500
Previously, when using Workers for Platforms’ dynamic dispatch API to send an HTTP request to a user Worker, if the user Worker threw an exception, the dynamic dispatch Worker would receive an HTTP 500 error with no body. When the dynamic_dispatch_tunnel_exceptions compatibility flag is enabled, the exception will instead propagate back to the dynamic dispatch Worker. The fetch() call in the dynamic dispatch Worker will throw the same exception. This matches the similar behavior of service bindings and Durable Objects.

​​ Headers supports getSetCookie()

Default as of2023-03-01
Flag to enablehttp_headers_getsetcookie
Flag to disableno_http_headers_getsetcookie

Adds the getSetCookie() method to the Headers API in Workers.

const response = await fetch("");
let cookieValues = response.headers.getSetCookie();

​​ Node.js compatibility

Flag to enablenodejs_compat
Flag to disableno_nodejs_compat
Enables the full set of available Node.js APIs in the Workers Runtime.

​​ Streams Constructors

Default as of2022-11-30
Flag to enablestreams_enable_constructors
Flag to disablestreams_disable_constructors
Adds the work-in-progress new ReadableStream() and new WritableStream() constructors backed by JavaScript underlying sources and sinks.

​​ Compliant TransformStream constructor

Default as of2022-11-30
Flag to enabletransformstream_enable_standard_constructor
Flag to disabletransformstream_disable_standard_constructor
Previously, the new TransformStream() constructor was not compliant with the Streams API standard. Use the transformstream_enable_standard_constructor to opt-in to the backwards-incompatible change to make the constructor compliant. Must be used in combination with the streams_enable_constructors flag.

​​ CommonJS modules do not export a module namespace

Default as of2022-10-31
Flag to enableexport_commonjs_default
Flag to disableexport_commonjs_namespace
CommonJS modules were previously exporting a module namespace (an object like { default: module.exports }) rather than exporting only the module.exports. When this flag is enabled, the export is fixed.

​​ Do not throw from async functions

Default as of2022-10-31
Flag to enablecapture_async_api_throws
Flag to disabledo_not_capture_async_api_throws
The capture_async_api_throws compatibility flag will ensure that, in conformity with the standards API, async functions will only ever reject if they throw an error. The inverse do_not_capture_async_api_throws flag means that async functions which contain an error may throw that error synchronously rather than rejecting.

​​ New URL parser implementation

Default as of2022-10-31
Flag to enableurl_standard
Flag to disableurl_original

The original implementation of the URL API in Workers was not fully compliant with the WHATWG URL Standard, differing in several ways, including:

  • The original implementation collapsed sequences of multiple slashes into a single slash:

    new URL("").toString() === ""

  • The original implementation would throw "TypeError: Invalid URL string." if it encountered invalid percent-encoded escape sequences, like

  • The original implementation would percent-encode or percent-decode certain content differently:

    new URL(" d%20e?f").toString() === ""

  • The original implementation lacked more recently implemented URL features, like URL.canParse().

Set the compatibility date of your Worker to a date after 2022-10-31 or enable the url_standard compatibility flag to opt-in the fully spec compliant URL API implementation.

Refer to the response_redirect_url_standard compatibility flag , which affects the URL implementation used in Response.redirect().

​​ R2 bucket list respects the include option

Default as of2022-08-04
Flag to enabler2_list_honor_include
With the r2_list_honor_include flag set, the include argument to R2 list options is honored. With an older compatibility date and without this flag, the include argument behaves implicitly as include: ["httpMetadata", "customMetadata"].

​​ Do not substitute null on TypeError

Default as of2022-06-01
Flag to enabledont_substitute_null_on_type_error
Flag to disablesubstitute_null_on_type_error
There was a bug in the runtime that meant that when being passed into built-in APIs, invalid values were sometimes mistakenly coalesced with null. Instead, a TypeError should have been thrown. The dont_substitute_null_on_type_error fixes this behavior so that an error is correctly thrown in these circumstances.

​​ Minimal subrequests

Default as of2022-04-05
Flag to enableminimal_subrequests
Flag to disableno_minimal_subrequests

With the minimal_subrequests flag set, fetch() subrequests sent to endpoints on the Worker’s own zone (also called same-zone subrequests) have a reduced set of features applied to them. In general, these features should not have been initially applied to same-zone subrequests, and very few user-facing behavior changes are anticipated. Specifically, Workers might observe the following behavior changes with the new flag:

  • Response bodies will not be opportunistically gzipped before being transmitted to the Workers runtime. If a Worker reads the response body, it will read it in plaintext, as has always been the case, so disabling this prevents unnecessary decompression. Meanwhile, if the Worker passes the response through to the client, Cloudflare’s HTTP proxy will opportunistically gzip the response body on that side of the Workers runtime instead. The behavior change observable by a Worker script should be that some Content-Encoding: gzip headers will no longer appear.
  • Automatic Platform Optimization may previously have been applied on both the Worker’s initiating request and its subrequests in some circumstances. It will now only apply to the initiating request.
  • Link prefetching will now only apply to the Worker’s response, not responses to the Worker’s subrequests.
Default as of2022-03-21
Flag to enableglobal_navigator
Flag to disableno_global_navigator
With the global_navigator flag set, a new global navigator property is available from within Workers. Currently, it exposes only a single navigator.userAgent property whose value is set to 'Cloudflare-Workers'. This property can be used to reliably determine whether code is running within the Workers environment.

​​ Do not use the Custom Origin Trust Store for external subrequests

Default as of2022-03-08
Flag to enableno_cots_on_external_fetch
Flag to disablecots_on_external_fetch
The no_cots_on_external_fetch flag disables the use of the Custom Origin Trust Store when making external (grey-clouded) subrequests from a Cloudflare Worker.

​​ Setters/getters on API object prototypes

Default as of2022-01-31
Flag to enableworkers_api_getters_setters_on_prototype
Flag to disableworkers_api_getters_setters_on_instance

Originally, properties on Workers API objects were defined as instance properties as opposed to prototype properties. This broke subclassing at the JavaScript layer, preventing a subclass from correctly overriding the superclass getters/setters. This flag controls the breaking change made to set those getters/setters on the prototype template instead.

This changes applies to:

  • AbortSignal
  • AbortController
  • Blob
  • Body
  • DigestStream
  • Event
  • File
  • Request
  • ReadableStream
  • ReadableStreamDefaultReader
  • ReadableStreamBYOBReader
  • Response
  • TextDecoder
  • TextEncoder
  • TransformStream
  • URL
  • WebSocket
  • WritableStream
  • WritableStreamDefaultWriter

​​ Durable Object stub.fetch() requires a full URL

Default as of2021-11-10
Flag to enabledurable_object_fetch_requires_full_url
Flag to disabledurable_object_fetch_allows_relative_url
Originally, when making a request to a Durable Object by calling stub.fetch(url), a relative URL was accepted as an input. The URL would be interpreted relative to the dummy URL http://fake-host, and the resulting absolute URL was delivered to the destination object’s fetch() handler. This was a mistake — full URLs were meant to be required. This flag makes full URLs required.

​​ fetch() improperly interprets unknown protocols as HTTP

Default as of2021-11-10
Flag to enablefetch_refuses_unknown_protocols
Flag to disablefetch_treats_unknown_protocols_as_http

Originally, if the fetch() function was passed a URL specifying any protocol other than http: or https:, it would silently treat it as if it were http:. For example, fetch() would appear to accept ftp: URLs, but it was actually making HTTP requests instead.

Note that Cloudflare Workers supports a non-standard extension to fetch() to make it support WebSockets. However, when making an HTTP request that is intended to initiate a WebSocket handshake, you should still use http: or https: as the protocol, not ws: nor wss:.

The ws: and wss: URL schemes are intended to be used together with the new WebSocket() constructor, which exclusively supports WebSocket. The extension to fetch() is designed to support HTTP and WebSocket in the same request (the response may or may not choose to initiate a WebSocket), and so all requests are considered to be HTTP.

​​ Streams BYOB reader detaches buffer

Default as of2021-11-10
Flag to enablestreams_byob_reader_detaches_buffer
Flag to disablestreams_byob_reader_does_not_detach_buffer

Originally, the Workers runtime did not detach the ArrayBuffers from user-provided TypedArrays when using the BYOB reader’s read() method, as required by the Streams spec, meaning it was possible to inadvertently reuse the same buffer for multiple read() calls. This change makes Workers conform to the spec.

User code should never try to reuse an ArrayBuffer that has been passed into a BYOB reader’s read() method. Instead, user code can reuse the ArrayBuffer backing the result of the read() promise, as in the example below.

// Consume and discard `readable` using a single 4KiB buffer.
let reader = readable.getReader({ mode: "byob" });
let arrayBufferView = new Uint8Array(4096);
while (true) {
let result = await;
if (result.done) break;
// Optionally something with `result` here.
// Re-use the same memory for the next `read()` by creating
// a new Uint8Array backed by the result's ArrayBuffer.
arrayBufferView = new Uint8Array(result.value.buffer);

The more recently added extension method readAtLeast() will always detach the ArrayBuffer and is unaffected by this feature flag setting.

​​ FormData parsing supports File

Default as of2021-11-03
Flag to enableformdata_parser_supports_files
Flag to disableformdata_parser_converts_files_to_strings

The FormData API is used to parse data (especially HTTP request bodies) in multipart/form-data format.

Originally, the Workers runtime’s implementation of the FormData API incorrectly converted uploaded files to strings. Therefore, formData.get("filename") would return a string containing the file contents instead of a File object. This change fixes the problem, causing files to be represented using File as specified in the standard.

​​ Experimental changes

These changes can be enabled via compatibility_flags, but are not yet scheduled to become default on any particular date.

​​ HTMLRewriter handling of <esi:include>

Default as ofTBD
Flag to enablehtml_rewriter_treats_esi_include_as_void_tag
Flag to disableTBD

The HTML5 standard defines a fixed set of elements as void elements, meaning they do not use an end tag: <area>, <base>, <br>, <col>, <command>, <embed>, <hr>, <img>, <input>, <keygen>, <link>, <meta>, <param>, <source>, <track>, and <wbr>.

HTML5 does not recognize XML self-closing tag syntax. For example, <script src="foo.js" /> does not specify a script element with no body. A </script> ending tag is still required. The /> syntax simply is not recognized by HTML5 at all and it is treated the same as >. However, many developers still like to use this syntax, as a holdover from XHTML, a standard which failed to gain traction in the early 2000’s.

<esi:include> and <esi:comment> are two tags that are not part of the HTML5 standard, but are instead used as part of Edge Side Includes, a technology for server-side HTML modification. These tags are not expected to contain any body and are commonly written with XML self-closing syntax.

HTMLRewriter was designed to parse standard HTML5, not ESI. However, it would be useful to be able to implement some parts of ESI using HTMLRewriter. To that end, this compatibility flag causes HTMLRewriter to treat <esi:include> and <esi:comment> as void tags, so that they can be parsed and handled properly.