That's exactly what the default styles are for: so you can see the document locally like it's going to appear in Roll20. There are a few icon images there that you'd have to download (and change the URLs in the CSS files) in order to view it 100% offline, but unmodified is good enough to do your testing outside of Roll20.
A good editor (like those recommended) will give you syntax-highlighting and auto-indent to help you see the document's structure at a glance, tab-completion to reduce your typing load, and general quality-of-life features like keeping track of which opening item (parenthesis, bracket, quote, etc.) corresponds to the closing one you just typed (to help avoid mistakes like "<a name=foo> href=bar>" or "a href=foo>"). Also, once your documents get big and complicated, things like regex searching can help you navigate better than a plain-text search can.
If you're looking for a WYSIWYG rather than a text editor, I know SeaMonkey has one bundled in with its browser (and the browser is pretty good too; I'm in the process of switching over to it from Firefox). I do my HTML and CSS stuff manually (and unattractively, but that's neither here nor there), so I haven't tried it lately, but I remember Netscape Composer (on which it's based) being reasonable back a decade or two ago when I last used it.
MULTI is an IDE aimed largely at embedded development (people writing software that runs on cars, planes, elevators, pacemakers, etc. Also Wii U games). As far as I know, it isn't sold individually -- only in bulk to businesses. But I really like the editor that comes with it; I don't know what I'll do when I change jobs and don't have access to it anymore.