I've been trying out to set the connectivity on my phone using TSProxy to slow down the connection to try to get stable metrics but I've seen another thing. When I look at the HAR file it looks like in practice on mobile (Chrome on Android), we will not start to render the page until the full HTML is downloaded:
Both the HTML and CSS got highest (or very high) prio but the CSS seems to always finish after the HTML is fully downloaded, so we cannot render before that. Meaning pages with a lot of HTML will be slower.
I verified by testing on Moto G4 on WebPageTest and it looks the same:
I know we touched this issue a couple of times before T125208 and at that time we talked about that the HTML had slightly higher prio than CSS (from Chrome) but now it looks that it has the same, but it still looks like the CSS never finish before the HTML is downloaded? Maybe file another upstream bug?
Upstream bug: => https://bugs.chromium.org/p/chromium/issues/detail?id=849106