Native apps are almost always preferable in this author's opinion, but if you're the kind of person who permanently keeps a browser open, then it can't hurt to have a few simple tools a bookmark away. This is a short list of favorites. Notably, most if not all are open source.
Text-based tools
- Browserpad is just what it sounds like: a plain text editor with almost no features, that auto-saves to local storage. It just works; that's what makes it useful.
- If you need a little more oomph, notepad.js.org provides an entire suite of little tools, including a rich text editor, task list and card-based note-taking app. [little tested]
Graphical tools
- To work with pixel art rather than writing, try Piskel. It's friendly and lightweight, with a balanced set of tools, and supports animations.
- For vector graphics, try SVG-Edit instead. Nowhere near as full-featured as Inkscape for example, but much lighter and faster despite running in a web browser, and capable enough. [little tested]
- Last but not least, Draw.io is a rich diagram editor with lots of features. Kind of clunky and with a learning curve, but probably worth the effort. It integrates with a number of online services such as Cryptpad.
Last but not least, Omni Tools is an ample collection of tools to work with everything from plain text, through CSV and JSON, to PDF and various image formats.