I don't think it's a bug. After all, browsers have very lenient parsers and if there's a syntax error (unterminated quote) in the style it shouldn't affect the rest of the HTML. The same goes for JavaScript: you probably still can't use the string literal '</script>' within an inline script.
Yep not a bug. Or at least if it's a bug, it's a bug in the HTML spec. But really I think it's pretty reasonable behavior.
Short answer is standard says to parse the contents of <style> elements as text.
Long answer is style start tag triggers [1] the generic raw text element parsing algorithm [2] which switches the parsers insertion mode to "text" [3] which only exits once an end tag is seen. Given that the generic raw text element parsing algorithm switched the tokenizer to RAWTEXT state [4] it will only output an end tag token from the RAWTEXT end tag name state [5] for an appropriate end tag token [6] which by definition will be "</style>"
> Or at least if it's a bug, it's a bug in the HTML spec.
Then again the HTML5 spec is based on what browsers already did, bugs included. Whether or not it's desirable behaviour or a bug is a moot point now. HTML is the way it is because of its history.
Think about the failure cases for `</script>` and `</style>` and it's obvious why the spec defines them that way.
If CSS and JS syntax were to decide when the end tags can appear, some malformed CSS or JS would break the end tag and cause the entire rest of the document to vanish as the browser wouldn't recognise it as HTML.
