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Google Pagespeed complains when you have blocking CSS in an external file. In HTTP/1 this probably makes sense, but what about now with HTTP/2 ?
If you inline critical CSS (above the fold), that bytes still need to download, parse and everything else, all before the document renders.
With HTTP/2, there is no need to make another connection since the same can be reused, so that is not an overhead. Plus, with server push you can even push the CSS file before it's being requested.
So... is inlining critical CSS still a recommended thing?
I agree that in heavy sites, you probably don't want to download all CSS. For example if you are visiting the gallery, you would only need gallery.css, not profile.css, not forum.css, etc. But that is manageable with chunks and other techniques (and still using external css files, no need to inline them)
Inlining also makes CSS not cacheable.
I am missing something?
This has nothing to do with the possible duplicate question. Whoever marked this as duplicated has no idea about what critical CSS is or probably didn't even have read this question.
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Before we get into what inlining CSS means and how to do it, it's important to first understand how CSS can affect performance. While JavaScript and images generally play a much larger role in negatively impacting performance metrics, CSS can also play a significant role.
Inline CSS means that the CSS is loaded in the <head> tag of the site's HTML. This is faster than the visitor having to download the CSS files directly from the server; however, if all the site's CSS is displayed inline it can actually slow down the load time of the entire site.
Inline CSS is considered useful as it reduces the number of files that the browser needs to download before displaying the web page. With an external CSS, the browser first loads an HTML file and then downloads the CSS file.
The main difference between inline CSS and external CSS is that inline CSS is processed faster as it only requires the browser to download 1 file while using external CSS will require downloading HTML and CSS files separately.
Since inline styling takes precedence over all other styles, one of the best times to use it is when testing or previewing changes and performing quick fixes on your website.
The goal of inlining critical CSS is to prevent a flash of unstyled content (FOUC). CSS is a render-blocking resource, meaning that it needs to be downloaded and parsed to create the CSSOM.
Yes it can still help. A lot.
When you download the HTML you still need to wait for that to download, then process it, and then make another request for the CSS file(s) and wait for those to download.
While downloading additional resources is quicker under HTTP/2, and there is not as much of a bottleneck when you have a lot of additional resources to download when using it, the CSS file still can't be requested until the HTML file has downloaded and been processed. Additionally the CSS file is usually prioritised by the browser, since it's render blocking, so usually will be one of the first resources requested meaning the avoidance of head of line blocking for HTTP/2 is not as beneficial for CSS resources.
When HTTP/2 Push becomes more common place it may not have as much impact as requests for the HTML page can also push the CSS file needed for that, but that's added complexity and there's still some questions as to how that will work (e.g. if the browser already has the CSS file then server should somehow know not to push it).
I wrote a blog post on this topic if you want more detail on this (and this is on a HTTP/2 site): https://www.tunetheweb.com/blog/inlining-css-is-not-for-me/ . I'm still not a big fan of this process as I explain in that post...
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