The upcoming Opera 9.5 will have an improved CSS support thoughFor example...
Opera doesn't support MathML at all.
It doesn't support APNG files (animated PNG).
It doesn't support color correction in PNG files.
It's missing lots of CSS2 feature support (eg, E:root, E:last-child), and even more CSS3 features (E:repeat-item, E:indeterminate, etc). CSS is actually the worst part of Opera these days. It's also missing overflow-x and overflow-y support (which every other browser has, including IE6). Opera doesn't support CSS3 backgrounds either. Or CSS2 direction (useful for right-to-left languages). Another biggy is CSS2 multicolumn layout (eg, newspaper-like text display), which exists in Firefox but not in Opera.
http://my.opera.com/desktopteam/blog/kestrel-is-coming