"Ideally I'd love to switch to an HTML5 site... alas, that's not going to be possible with this particular site."
This doesn't mean you cannot use the html5 doctype (<!doctype html>), which is backwards compatible and is always the html-strict (pure standards) mode; i.e. as long as you don't use the special htlm5 tags and features it will work in all IE-versions above IE5.5.
As far as I know, the following things are happening.
For the best cross-browser results a strict doctype is recommended: the html5 doctype or the strict variants of html4.01 or xhtml1.0 (see here).
If this is impossible in your case (too much work or other ways; f.i. use of the target attribute ), you can use the XHTML 1.0 Transitional variant as well.
- With setting the html-validator option (by hand) on the strict doctype, you can try to get the code as much strict as possible: that doesn't hurt.
"IE=edge" tells Internet Explorer to use the highest rendering mode available for the version of IE of the visitor: that will be o.k.
All this will be the "Document Mode". Once chosen, you have nothing to do with it.
With the "Browser Mode" button you can test (simulate) the results as if it was an older IE-version.
- If something is not supported in a certain IE-version (for example a css rule), you can try to find a workaround and make a Conditional Comment to serve this version.
Some more info at MSDN: Testing sites with Browser Mode vs. Doc Mode