Here is a collection of diagram, chart, flowchart, whiteboard and similiar editors. Some of these appear to be difficult to search on internet search engines. Ideally, they should be able to, but I am at the moment unsure of it they can, run on a webbrowser client usin javascript togheter with SVG, Canvas, or similiar techonogy. Note that the description is subjective from this repository author, it might differ from actual fact. Is anything is incorrect, please file an issue.

Dgrm.net
https://github.com/AlexeyBoiko/DgrmJS
A SVG based editor that at the core is rather limited. You can add shapes, move them, change color, add text and connect shapes with pathes. Everything else such as Undo / Redo, resize, group select, collobration is pro only. It is not documented at the moment.

tldraw
https://github.com/tldraw/tldraw
A Canvas based whiteboard editor that is build with React? It is possible to create plain looking flowcharts, that looks like drawn on whiteboard and has full functionality to edit. It appears to be customizable and extendable.

excalidraw
https://github.com/excalidraw/excalidraw
A more sophisticated editor found that appears to have more features. The emphasis is clear to mimic handdrawn, paper like style graphics. Through, it is possible with a few adjustments to make it look more professional. While extendable, it appears to be rather complex.

drawio
https://github.com/jgraph/drawio
The most full featured editor with a more classical old style look. There are no collbration feature in the source code and is intended to use as-is. It is not recomended to use as a library or extend upon it.

Forgot anything? Please file an Issue or make a Pull request if there are any other similar editors!