Treating DOM as the state of my JavaScript code instead of storing state separately and then synchronizing.

Using HTML instead of parsing Markdown by applying white-space: pre-line with some additional styling for semantic tags.

Styling style and script tags as visible to embed examples into HTML pages without duplicating or entity-encoding them.

Using files as a key-value store. (File name = key. File content = value.)

Using folders/files as a one-to-many messaging bus. E.g. one app creates empty do-something.txt file in a folder, couple other apps monitors the folder and do stuff. Very simple setup, but allows to create multiagent systems in different languages with persistence and easy way to debug/interact with the whole thing manually.

Using inheritance instead of mocking frameworks. (Instead of mocking something out, extend the class, override "mocked" behavior and test that class.)