While working on a SharePoint 2013 branding job we noticed the layout of some pages was off only in Internet Explorer (10 and 11). We used various new HTML5 semantic elements like main, section and article while the final portal would run on Internet Explorer 11. To our suprise we found that the <main> element as a main container of the Page Layout was not working with max-width of a centered Page Layout.
After some searching on the web I found that all HTML5 semantic elements are supported by all modern browsers except for one little company called Microsoft that supports all elements except for the main element. Brilliant.
http://caniuse.com/#search=main
Does not include support for the <main> element.
Solution: main {display: block; }