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CSS is render- but not parser-blocking. This means that while a browser is fetching CSS, it is permitted to continue parsing the rest of your HTML but not render it. HOWEVER! If you’re not careful, you can inadvertently end up in parser-blocking territory.

Now let’s introduce one little change. A script with a single space character immediately after the stylesheet. What happens now?

The Parse HTML tasks are bolted onto the end of the CSS’ fetch. The main thread wasn’t allowed to do anything else while the CSS was in flight.

They are interleaved with rendering tasks, which shows is the browser’s ability to progressively render HTML, but one space character held up the main thread completely!