The exclamation mark means that your rule is forced. Therefore, everything will redirect to /index.html when you have the /* /index.html 301! rule… even your style sheet! This is something known as shadowing.
The /* /index.html 301 rule, without the exclamation mark, will redirect anything to /index.html when the requested file/page doesn’t exist.
This little guide may help and covers some of the topics such as fallback rules, shadowing and order of execution.