HTML/CSS is a small piece of a stack that is fairly easy to pick up. There are hardly ever reasons to hire someone who is FE only (at small companies) and I often find that people who are FE only don't have the kind foundation that would allow them to venture into complicated JS code.

The author seems to be implying that SEs given front end responsibilities will struggle with them but that's only true in the way that they struggle with anything else new (the struggle is short lived because we're talking about markup with quirks).

Also, OP, if you read this, your footnotes link to the original text but it's covered by your huge fixed header.