In HTML the markup os accomplished by surrounding an item by tags. This is emphasized. However, this would be a slight violation of the principle logical document design because we are saying what the text should look like . In HTML the big thing is hyper links and there is an elaborate code for them. Both of these plain TeX conventions violate the custom that delimiters or tags should come in left and right pairs.
More complicated environments allow alignment of multi-line equations, matrices, etc.
You can see a TeX document containing all the formulas described here.