Check out carbone.io
It’s not super powerful but does the trick for basic stuff. They have both self hosted and a cloud/paid version.
pspdfkit.com might also have something for rendering PDFs as a service.
If you are looking for a quick way to get up to speed with flexbox or css-grid you should check out
https://flexboxfroggy.com/
and
https://cssgridgarden.com/
They are fun little tutorial games that introduce the basics
A heat-map of sorts for a codebase showing the files that have the most-frequent/most-recent edits as hottest. Purpose being to help acclimate new developers to a project by easily seeing which parts of the codebase are most actively worked on.