In the process of editing a particular step on Stepik you can go to HTML editor (button "Show HTML").

It is allowed to use the following tags:

[ 'a', 'bold', 'reduction', 'audio', 'b', 'Quote', 'br', 'code', 'div', 'em','H1', 'H2', 'B3', 'I', 'IMG', 'Li', 'ol', 'p', 'before', 'span', 'strike', 'strong', 'ul',]

and their attributes:

{ 'a': ['too', 'Title', 'Rel', 'target'], 'bold': ['name'], 'acronym': ['name'], 'audio': ['source', 'controls'], 'div': ['class'], 'img': ['source', 'audio', 'class', 'Header', 'width', 'height'], 'span': ['class'], 'p': ['class'], 'code': ['class']}