[whatwg] Media aware JS loading

[Yoav Weiss]

>
> How bad is the current situation with JS-based loaders? I would expect a
> JS-based loader and the couple of feature-detection tests to be rather
> small as well as heavily cachable, so not that awful for performance.
>

JS based loaders have several drawbacks:
* JS files are not discovered by the preloader, which means the browsers
can't prioirtize their download appropriately.
* They require JS inlining, which will become significantly harder with CSP
(JS nonces make it possible, but it'd require server side logic)
* Arguably, attributes make authoring easier, which may mean authors are
more likely to avoid useless JS downloads.


> I'm not sure I understand your point about preloading scripts parsing. Do
> you want them to be preparsed (which costs in battery as you mention) or
> not preparsed (which will result in longer time when actually needing the
> script)?


I was referring to HTML parsing (so the discvery of JS resources in HTML),
not the parsing/execution of the JS resources themselves.

 How much time can be expected to be gained from such a feature given the
> state-of-the-art JS loaders?
>
> Overall, what is the expected gain between a JS-based loader and a
> declarative media-aware loader? 1ms? 100ms?
>
> While the expected gain may vary per site, I'd estimate that it's probably
in the area of hundreds of ms for most sites. A recent study by Google
(addressing this specific issue) showed that the PreloadScanner gives ~20%
improvement https://plus.google.com/+PaulIrish/posts/WcsqfSFZAfR