Before checking the CSS or HTML, ensure that users do not have Compatibility View set. This can cause with site to appear to render in IE7 (Quirks mode). This is either set at the browser, or set by an IT administrator across a network. If you go in to your settings gear icon, click on Compatibility View Settings, see if your website is added to the textbox OR if "Display intranet sites in Compatibility View" is checked. If so, remove/uncheck.
If you don't see that, check with your IT administrator, they can globally set computers in network to compatibility view - this is common.