DesignPLUS User Guide
If your institution has CIDI Labs (a plugin for Canvas), you should check out these guides. In particular, I recommend enabling the "advanced" features.
Canvas Style Guide
This page details the CSS class names you can use in Canvas. The style guide is no longer being updated BUT after some researching, it seems the class names included will continue to work in Canvas for the foreseeable future. Note that form elements (e.g. ) are stripped from the content editor so you can't really use the HTML in the "Forms" section of the guide.
Inline CSS that's stripped in Canvas and Brightspace
This is a spreadsheet where I keep track of CSS that's stripped from Canvas and Brightspace.