The first user-visible part of a Web document is referred as "above the fold" (referred as "first screen" in the Submission), similar to the visible upper half of the front page of a folded newspaper. The Submission proposes a hint that the author can use to indicate if a Web browser could accelerate the rendering fo the document for the part which is "above the fold"
The optimization technique relies on, but is not limited to, adjusting the buffer size for the tokens generated by the HTML or XML parser tokenizers, as well as taking into account the height of the Web content and the screen. Since some Web applications makes heavy use of dynamic resources, the optimization technique could cause painting disorder or performance regression and could therefore be disabled by the author if needed.
While the Submission has the advantage of giving an easy choice between two different rendering strategies without requiring changes in the developer code, it is important to note that it could be misused by the author if not understood and applied properly, especially given the wide range of screen sizes in devices nowadays. As such, introducing optimization hints in the Open Web Platform could backfire on the future performance of the Web.
There are alternative existing performance practices around progressive rendering in order to optimize the "above the fold" rendering with HTML and CSS to avoid blocking the rendering the page, such as inlining critical CSS and doing asynchronous loading for "below the fold" resources. Those practices do require however finer grain changes by the developer.
This work is welcomed and is in scope as an input to the Web Performance Working Group. As such, it should be considered there. The Group is currently discussing alternative resource hint and priority approaches as well (Resource Priorities and Resource Hints).
Continuing discussion of this topic, including whether or not this particular optimization technique is ready for W3C standards-track work, is welcome via firstname.lastname@example.org [archive].
Disclaimer: Placing a Submission on a Working Group agenda does not imply endorsement by either the W3C Staff or the participants of the Working Group, nor does it guarantee that the Working Group will agree to take any specific action on a Submission.Philippe Le Hégaret <email@example.com>