I would echo what Josh is saying. There are some tricky, hard to see errors that break things.
For links, lately I have had a lot of students not type the file name in exactly correctly. mixed cased letters is a big one. Also not putting in, or having a malformed end tag has been another common hard to see one.
In other errors
I have found recently that spaces between the attributes in tags prohibit things working. Like
<img src="helpMe.jpg"alt="My code doesn't work" />
See how there is a space between the src and the alt. That has been a real pain for me lately.