Here's a fantastic explanation: http://diveintohtml5.org/semantics.html#unknown-elements http://diveintohtml5.org/semantics.html#unknown-elements The TL;DR is

Here's a fantastic explanation: http://diveintohtml5.org/semantics.html#unknown-elements http://diveintohtml5.org/semantics.html#unknown-elements The TL;DR is that all browsers treat unknown elements as inline elements, so for older browsers you'll need to explicitly define elements like header, section, etc as display: block. IE has an extra wrinkle, where it won't apply CSS styles to unknown elements. The fix, which Modernizr applies, is to create dummy elements of each new type in JavaScript before they appear in the page. Once that's done, IE happily applies styling to the new elements.