HNEd

This is the "Hypertext Notes Editor". It's just a page to make it easier to edit HTML for my college notes. Before I would jot things down on pen and paper, but I would lose notes over time, hard to undo, and hard to copy/share and make backups. I tried LyX because it had a nice equation editor and could export to different formats, but I found that it was really difficult to style (for submitting homework) and wasn't as extensible as HTML/CSS/JS. With the Adoption of MathML across all major web browsers, and the contentEditable feature, this was really easy to write as a static HTML file. 

Use a little bit of plain javascript contained in a single object, that is easily removable. sematic HTML. fully editable without a mouse just using the keyboard shortcuts, or on mobile without keyboard shortcuts.

The only caveat is that it doesn't support making documents with a nested tree structure, to keep editing simple. There is only one level of divs that get inserted under the <body> when you add them using the bottom toolbar. Under that level of divs, there is another level of divs produced by your browser whenever you press enter within one of those divs; This is just how contentEditable works. Underneath that you can put <span>s. To edit the spans, you can unfold all the spans on a <div> using, and then merge them back with sadasd.

It can convert to latex if you want, but I don't know if it has complete support for everything. I've only checked the basic things I use: Text, headers, equations, images, embedded files, and such.

When you want to save
Keyboard shortcuts.