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In this example it is telling the HTML site to make anything with the tag follow the formatting.
h1, h2 etc. - anytime these tags are used the font family, size, weight, variant, text align, margin top/bottom, (settings you choose) will always be the same.
An example of a custom set of code modification would be;
Example:
.text { font-size: 1.6em;
.test-area {margin-center: 1.0em;
These custom tags would be called in the HTML code section you would like the changes to affect.