lhooper

OK, I resolved this. Here is the explanation if anyone else happens to have the same issue:

In editing single.php, I added a <!-- comment --> at the very top of the file, before the_post(). This was strictly for my own benefit, to remind me what the file did.

Anyway, other browsers didn’t mind this. But IE reacted by (it seems) changing its interpretation of markup/CSS throughout the entire page — in particular, affecting the widths of various things. I don’t understand why this would be the case, but removing the comment solved the issue entirely.

The links above no longer illustrate the problem, but hopefully the explanation is enough to help someone else who (however naively) has added a comment at the top of single.php.

Thank you, esmi, for looking & responding. I’m sure the various things noted by the validators aren’t helping — and I apologize for pointing folks at sloppy work — but almost none of the validation errors are unique to the page in question, and they’re not breaking other pages. That’s what led to my query. The behavior itself is strange, I think.