All attributes must be in quotation marks

In HTML, you don't always need to put quotes round attribute values. Values with spaces (like font names) or special characters (like URLs) need quotes but most one-word or numeric attribute values don't. You could put them in if you liked but it didn't matter much one way or the other.

XHTML won't stand for any of that nonsense: you must put every attribute in double quotes. There are no exceptions. So:

This could easily be a pain in the neck if you need to convert old HTML documents.