I wouldn't do anything to "correct" your guide - I think it is "correct" as is. This comment is great for its informational content but I'd consider it an addendum, not an erratum.

If you like it might be nice to include a section on historical and/or niche browsers that lack some of the elements this guide describes - like e.g. Dillo which is a modern browser that supports HTML4 & doesn't support Javascript. But your guide should (imho) centrally focus on the common expectation of how popular browsers work.