> there can be information critical to styling a webpage that lives outside of the HTML alone
There... shouldn't be? I'm kind of missing the part that explains why you'd need this? Are there any concrete examples where "I need the headers and/or URL" lets you do something that you can't already do by making sure your document has the information it needs, in the document itself? (if you need URL-specific CSS, you just... load more CSS...? You template in the necessary <link> elements because your server knows which URL is being requested?)
There... shouldn't be? I'm kind of missing the part that explains why you'd need this? Are there any concrete examples where "I need the headers and/or URL" lets you do something that you can't already do by making sure your document has the information it needs, in the document itself? (if you need URL-specific CSS, you just... load more CSS...? You template in the necessary <link> elements because your server knows which URL is being requested?)