WebStorm autocomplete html code

Such "transformation" (abbreviation expansion) called Emmet .. and by default it uses Tab key.

But it can also be Enter or Space .. or even custom key of your choice -- you can change it at Settings/Preferences | Editor | Emmet

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P.S. For big chunks of code -- have a look at Live Templates (Code Snippets).

You can use the Tab key to auto-complete the code. For example:

Write .row#row and press Tab key will make <div class="row" id="row"></div>.

I think this would help.