What a surprise today, a HTML5 page is loaded in IE9 in so called QUIRKS mode as default (instead of standard) which breaks pretty much everything on the page. What is the quirk mode? Found on internet:
Quirks Mode is a mode of operation of web browsers such as Internet Explorer (IE), Firefox, and Opera. Basically, Quirks Mode (also called Compatibility Mode) means that a relatively modern browser intentionally simulates many bugs in older browsers, especially IE 4 and IE 5
This happens especially if there is some comment above the first html tag
<!DOCTYPE html> in document. To fix this we have to tell IE browser "Behave like a modern browser mate!" by adding a meta tag in HTML header:
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
What to add, there is always something about IE browsers.