As a professional web developer I can say that Voyager supports more of the badly designed html than IBrowse does. However the stability factor and the amazing javascript support in IBrowse (JavaScript v1.4 without CSS), I would have to say IBrowse any day of the week. Although, if you use MorphOS or plan to use MorphOS, I would sincerely think about going with Voyager.
Now, whichever you go with, make sure you grab a copy of AWeb APL. I've found AWeb to be the most html4.01 complaint out of the other two. There are a few things AWeb has problems with (like xhtml doctype identifiers or the background="" table cell tag) but it's only cosmetic.
Another thing, Voyager has some of their CSS engine written, just not tied to the rendering engine just yet. I'm sure when MorphOS is updated along side Voyager, there will be support in there for CSS. I don't know when any of the browsers are planning on supporting DIV or SPAN tags. It seems the DIV and SPAN tags are the most troublesome tags out of the html4.01 secification. Even though they were introduced in html4, there are already some attributes of those tags that are depricated as of html4.01 with reference to CSS1 type style tags.
:pint: