What CSS implementation problems and bugs are there?
Internet Explorer 6
When using auto for setting margins, IE6 only supports this when running in standards compliant mode (not quirks mode). IE6 doesn't support the attribute selector (for example input[type="submit"] { ... }).
See MSDN: CSS Enhancements in IE6 for details on how MSIE 6 now handles this [padding] according to spec, if you use a magic word on the first line of your HTML document.
Internet Explorer also doesn't support position: fixed, although you can kludge together a work-around using Javascript (It cannot be taken for granted that Javascript is enabled or available in browsers, so sensible default positioning applies.):
<style>
#moo {
position:absolute;
top:expression(+(document.body.scrollTop)+220+"px")
left:expression(+(document.body.scrollLeft)+220+"px")
}
</style>
<body onscroll="document.recalc()">
<div id="moo">Moo</div>
Netscape Navigator 4
Netscape went their own way in the implementation of stylesheets, delivering Javascript Style Sheets (JSSS). When the CSS specification was released by the W3C, they tried to adjust the style sheet language to match CSS. As a result of this switch and the increasing attention poured into Mozilla - a standard's compliant browser engine, the CSS capabilities of Netscape 4 are not well implemented, and there's not much hope of it being corrected.
Netscape 4 tends to crash when using clear in a style sheet.