Today’s reminder from your friendly neighborhood codeslinger is "don’t use underscores in css ids!" You know, those lines _ that programmers like to use instead of spaces in variable names (because you can’t really have spaces in a variable name in most languages). This was an illegal character in the original CSS1 and CSS2 specifications (updated in 2001 to be valid) so setting an id to foo_bar (e.g. <div id="foo_bar">) would be translated by the browser as foo\_bar (e.g. <div id="foo\_bar">), well, some browsers, not all browsers because you know browsers are so consistent in their interpretation and implementation of these so called web standards.
The solution? Instead of underscores, opt for dashes (e.g. <div id="foo-bar">) or camel case (e.g. <div id="FooBar">) but remember that id’s and class names are case-sensitive.