Do you know that the minimal font-size that can be set in CSS and will be supported by Opera is 10px? I mean that font-size: 8px;,font-size: 9px; and font-size: 10px; will look in Opera the same. Not a good thing for a good browser I think. Firefox supports even 1px (of course, you’ll read nothing, but it supports that!). Internet Explorer also supports font-size of 1px as well as Apple Safari does. Google Chrome does too. So here is a table that accumulates these data. If I have experience with new browsers, I’ll extend the table.

Browser Supported minimal font-size
Opera 10px
Apple Safari 1px
Mozilla Firefox 1px
Internet Explorer 1px
Google Chrome 1px

Maybe Opera’s purpose was just to make the life of black hat search engine optimizers more difficult?