Random keywords are not useful as tags. They're often just used to describe the problem using tags instead of the question itself. Some keywords are also bad as tags, because they can mean different things in different context.
In my opinion a keyword is good as a tag if there can be an expert on the keyword who can answer almost any question related to it. Hence, I consider tags such as alert to be pretty bad. An alert can be almost anything that is shown to the user, and how the alert is done and what the alert actually is depends a lot on the website/program and even the underlying technology and language used.
Generally, if I can imagine myself searching for questions to answer using the tag, it's a good tag.