This is the execCommand interface, common to Internet Explorer , Firefox, Chrome, Safari or Opera. Except that it's not common because they all generate different HTML in response to these commands.
<BLOCKQUOTE style="MARGIN-RIGHT: 0px" dir=ltr>in IE8 (doesn't even quote all the attributes!)
<blockquote>in Opera 11
<p style="margin-left: 40px;">in Firefox 3.6
So Opera 11 looks like the winner on generating HTML to web standards. The others are all an unholy mess. Internet Explorer's output frequently will not validate. Chrome and Safari insert unnecessary classes and, together with Firefox, use inline styles which will trample all over your carefully crafted stylesheet rules.