I feel that the tags have been merged in the wrong direction. semantics is a noun, whereas semantic is an adjective. In my opinion, adjectives are usually unsuitable for tags, and nouns should be preferred. The one exception I can think of to this rule is when the tag describes the question itself (i.e. subjective), rather than what the question is about.
To summarize, the semantic tag suggests that the question is semantically correct, i.e. no grammar problems, whereas the semantics tag suggests that the question is asking about how to code semantically. I think it's pretty obvious which one is tag-deserving, and which isn't. So please can we swap the two tags around!
Edit: Even the tag wiki seems to think the tag is called "semantics":
The semantics of programming languages and other languages is an important issue and area of study in computer science. Like the syntax of a language, its semantics can be defined exactly.