First of all, it takes almost no time at all to learn HTML/CSS. I'm willing to bet that it takes just about as long as it takes to learn how to use all the controls in Muse. Supporting IE6 is the most time consuming aspect of cross-platform web support, and there is plenty of precedent for that available freely online. ASM is a not so easily learned and understood as HTML/CSS.
Second, HTML is for telling the browser how to lay out content. When you use Muse or Dreamweaver or iWeb or whatever, you're essentially "scripting" your HTML in a proprietary GUI. When that GUI changes or disappears, how will you maintain that page? Yes, by hand.
All WYSIWYG GUIs should seek to output human-maintainable code, at the very least. It isn't a performance issue.
Second, HTML is for telling the browser how to lay out content. When you use Muse or Dreamweaver or iWeb or whatever, you're essentially "scripting" your HTML in a proprietary GUI. When that GUI changes or disappears, how will you maintain that page? Yes, by hand.
All WYSIWYG GUIs should seek to output human-maintainable code, at the very least. It isn't a performance issue.