This is a conditional IE statement. They are only read by IE. Any other browser will read them as any normal comment, note the at the beginning and end of the statement.

The X-UA-Compatible meta tag allows web authors to choose what version of Internet Explorer the page should be rendered as. IE11 has made changes to these modes; see the IE11 note.

When Modernizr runs, it removes the "no-js" class and replaces it with "js". This is a way to apply different CSS rules depending on whether or not Javascript support is enabled. See.