Is there a widely accepted, precise standard for this like there is standard for the C language?
Obviously, I am no mathematician :-)
People ask for this from time to time, but the thing is that mathematical notation really isn't a very big deal for anyone who has taken the time to study the concepts. By far the biggest problem is learning to understand the concepts that the symbols represent. Situations where someone is familiar with the underlying concepts, but unaware of the conventional notation are, needless to say, rare. Feynman apparently ended up in this situation after discovering a lot of mathematical facts on his own, and later had to learn the names and notation that everyone else was using. But it's not a problem that most people will ever have.
As others say, there is no standard, and conventions vary by subfield, publication, author and over time. This is esp. true at the research level, where the mathematical content is still being worked out. Subfields have certain conventions, and well-written books and papers will normally introduction notation or include an index of notation, esp. if the notation is novel or they different from the usual conventions. You could start compiling something like this by going through the standard undergrad and grad textbooks for each subject.
For another example, one of my Measure Theory textbooks denotes outer measure of a set A as |A| whereas another one uses m*(A).
Consider the following partial list of symbols that occurs at the beginning of Dummit&Foote's Abstract Algebra[0]. This tells you nothing unless you know the underlying ideas these symbols represent. This list is there to tell you what kind of conventions they would be following because different symbols might be used in other books.
but still quite standard in term of it's LaTex
There're varieties of it
e.g MathJax for online site like Quora, on others may differ