This is to support content negotiation use cases such as differential serving of variable fonts, color vector fonts, responsive images, and other third-party content which requires client information lost by user agent reduction (an ongoing project). For example: variable fonts allow significantly less font information to be transferred without loss of functionality, but only works on specific operating systems.

It’s already possible to set a Permissions Policy in the HTTP response header, but for sites without the ability to modify HTTP headers a HTML solution would be ideal. This proposes a meta tag which allows delegation of client hints to third-party origins. These tags could be included in code-snippets for embedded third-party content for ease of use.

For example, to specify third party requests to https://foo.bar must include sec-ch-width you could include:
<meta name="accept-ch" content="sec-ch-width=('self' 'https://foo.bar')">

You may still omit the permission policy and rely on the default allowlist as follows:
<meta name="accept-ch" content="sec-ch-width">

Note that this is the equivalent of the following today:
<meta http-equiv="accept-ch" content="sec-ch-width">