What does it mean when a CSS rule is grayed out in Chrome's element inspector?

Question

I\'m inspecting an h2 element on a web page using Google Chrome\'s element inspector and some of the CSS rules--which appear to be applied--are grayed out. It

Answer 1

I'm answering long after the question already has many correct answers because I encountered a different case of having a block of CSS code greyed out and uneditable in Chome DevTools: The block in question contained U+200B ZERO WIDTH SPACE characters. Once I found those and removed them, I could edit the block in Chrome DevTools again. Presumably this might happen with other non-ascii characters as well, I haven't tried to figure that out.