Do not assume that a change or removal is impossible without first checking.

<h3 id=removing-features>Remove or change capabilities only once you understand existing usage</h3>

Prioritize compatibility with existing content when removing or changing functionality.

Once a significant amount of content has come to depend on a particular behavior,

removing or changing that behavior is discouraged.

Removing or changing features and capabilities is possible,

but it first requires that the nature and scope of the impact on existing content

is well understood.

This might require research into how features are used by existing content.

The obligation to understand existing usage also applies to any features that content relies upon.

<h3 id=removing-features>Prioritize compatibility when changing or removing features</h3>

Before changing how a feature behaves,

understand how websites are currently using it.

Gaining a good understanding might require research,

for example by adding metrics to a widely-used user agent

or by [searching the HTTP Archive](https://har.fyi/guides/getting-started/).

Breaking content harms users,

and the benefit of a change has to

significantly outweigh that harm to be worth doing

[[the-web-is-unversioned]].

The obligation to understand existing usage

also applies to non-standardized or unspecified features

that content relies upon.

This includes vendor-proprietary features and

behavior that might be considered implementation bugs.

Web features are not solely defined in specifications;

they are also defined by how content uses those features.

implementation bugs.

Despite this, it is sometimes acceptable

to break some existing content

in order to improve the experience for a large number of web users.

Breakage is more likely to be acceptable if:

* Only a tiny amount of existing content depend on the feature or behavior.

* Only a tiny number of people see that content.

* The content appears only in test cases or examples.

* The content is already broken in some widely-used user agents,

and either they have not received many bug reports about it,

or the change is expected to improve interoperability.

* The benefit of breaking content is very large.

For example, removing old SSL and TLS versions caused a number of sites to become unreachable

but prevents more security breaches.

<h3 id="leave-the-web-better">Leave the web better than you found it</h3>