@dgrammatiko System test is failing at the same place all the time, doesn't look like the usual unrelated issues. Could you check https://ci.joomla.org/joomla/joomla-cms/40988/1/22 ?

@richard67 this is just a working POC. I'm not expecting all changes here to land as a single PR. Basically what matters here are the changes in the stylesRenderer and scriptsRenderer, everything else is just code that bothers me (eg scripts to bottom) or just code to showcase that this is actually working.

4c43cb3 is a shorter more focus version. The main point here is 4c43cb3#diff-2ede9f6c2a9f8106b0c8ccb140ba0c53999c95e84f8f732702ad700589ad7bdf and 4c43cb3#diff-16733093e0a3baea0a441e3c42154e63cb2a9e01c4738992d79839e2c3d2e715

@richard67 tests are passing but people seem against this PR ?

... but people seem against this PR

@dgrammatiko Thumb down from Sharky and ... from yourself? The latter confuses me a bit.

The latter confuses me a bit.

I'm sick and tired of getting a thumb down on every PR I do so I thought I should follow the trend ?

I'm sick and tired of getting a thumb down on every PR I do so I thought I should follow the trend ?

Ah, then it's ok. I worried a bit you could develop a dissociative identity disorder ?

hold on, here is much of everything ?

Please remove inlined option, it a questionable topic, it is better to do separately.
Please revert changes for XHTML, as long as Document may render XML, it should stay.
And revert "conditional", I know it is old, but as long as it not used, it does not hurt to have, and may be still useful for someone. It is better to do separately.

Now about the lazy CSS
I would suggest to do it with help of attributes only, does not need extra option lazy.
We only need to fix that StylesRenderer to accept a custom rel attribute

Check this example:

<link rel="lazy-stylesheet" href="styles.css">

<script type="module">
document.head.querySelectorAll('link[rel="lazy-stylesheet"]').forEach(function($link){
  $link.rel = "stylesheet";
})
</script>

So in the Style asset need to add attribute rel:"lazy-stylesheet", and that all.

I thought we can combine it with data-defer similar to scripts, but now I do not think that it makes sense.
Custom rel attribute should be enough.

I would suggest to do it with help of attributes only, does not need extra option lazy.

I like it, so if we agree on this then I will add the code for this (only the lazy-stylesheet) in the other PR.

About the other changes: at some point, we have to discuss what we deliver, conditionals are not supported in any of the Joomla 4 supported browsers and XHTML is dead since 2012 so I have no clue why we have to have those conditionals there.
I did those changes just to (hopefully) open the discussion

About the inlining part, this is something that the CMS should provide as it's easy to code and quite helpful in many instances

In short, I'll take the lazy-stylesheet to the other PR but it would be nice to have some comments on the other issues I raised here