Late loading of webfonts can cause layout instability and late text rendering. By default, a fallback font gets rendered in place of the webfont which gets replaced when the font loads and causes elements to move depending on the differences between the fallback and the remote font. The objective is to prevent the page elements to move around because of this behavior.
Introduce a new feature for document policy, namely, font-display-late-swap to control the value of font-display descriptor (more info on the font-display here). When the feature is disabled in a document, all the font-display values will be restricted to optional for that document (except for when explicitly set to fallback).
A page can use the font-display-late-swap feature by specifying it in its HTTP header:
Document-Policy: no-font-display-late-swap
which would disable the feature on the page. Alternatively, it can be set by modifying the policy attribute in the iframe tag. for example:
<iframe src="https://example.com" policy="no-font-display-late-swap"></iframe>
would disable the feature in the embedded frame.