In CF10, if you used cflayoutarea on a page, you could expect a couple classes added to <body> and <html> - namely, ext-webkit, ext- (your browser) to <body> and ext-strict to <html>. These are pretty specific classes and don't interfere with much.
In CF11, cflayoutarea adds the x-border-box class to your <html> tag, giving your entire page the following style from ext-all.css:
.x-border-box,
.x-border-box * {
box-sizing: border-box;
-moz-box-sizing: border-box;
-ms-box-sizing: border-box;
-webkit-box-sizing: border-box; }
The potential pool of things with which this will interfere is much larger.
May I ask why CF does not just add the necessary styles to the top level <div> it uses for cflayoutarea? If I remove the CSS from ext-all, then cflayoutarea looks wrong. I know that I could go in and copy those styles to one of the classes on the parent <div> generated by cflayoutarea, or the parent <ul>, but then I'm monkeying around with CF's ext-all.css for every CF install we work with.