When you apply a width to an ol or ul, Explorer Windows secretly applies more rules. Moreover, 5.0, 5.5 and 6.0 each apply different rules.

(note: This bug is very similar, but not identical, to Ordered list rendering with width on LIs.)

Test page. Workaround is not included.
Reported by ppk.

When you navigate with the Back and Forward buttons, Opera doesn't fire any load and unload events.

Test page. Workaround is not included.
Reported by ppk.

IE applies HTML normalization to the data that is assigned to the innerHTML property. This causes incorrect display of whitespace in elements that ought to preserve formatting, such as <pre> and <textarea>

Test page. Workaround is not included.
Reported by Sebastian Redl.

When you define visibility: hidden on * Explorer Windows and Safari don't show anything ever again.

Test page. Workaround not included.
Reported by Giuseppe Bertone.

When you remove the elements of a form, Safari nonetheless keeps references to these removed elements in the document.forms.elements array.

Test page. Workaround is not included.
Reported by Petr Stanicek.

If both a strict XHTML doctype tag and an XML prolog are written into a popup using separate document.write commands, Mozilla/Firefox/Gecko based browsers will revert to quirks mode, instead of standards compliant mode.

Test page. Workaround is not included.
Reported by TarquinWJ.

font-weight: 600 and font-weight: bold are not equivalent. The browsers stretch up their fonts when you use 600. For a really correct bold font you must use bold or 700.

Mozilla exhibits a similar bug only on Mac: any text with font-weight: 600 appears as normal, non-bold text.

Test page. Workaround is included.
Reported by ppk.

When you change the font size of button elements, odd things start to happen in IE Win & Mac, and Mozilla. Of course, each browser has its own take on 'odd things'. The Mozilla problem is solvable.

Test page. Workaround is included.
Reported by ppk.

In certain situations Opera seems to copy the padding-top to the padding-bottom.

Test page. Workaround not included.
Reported by myf.

Handling of innerHTML in XHTML pages (with MIME type application/xhtml+xml) is weird. Getting works, setting doesn't.

Test page. Workaround is not included.
Reported by ppk.

Up to XP SP2 Explorer 6 Windows supported @i url(some.css).

Test page. Workaround is not included.
Reported by ppk.