Togetherness Table is an open source VTT aimed at being as developer-friendly as possible.
From the Github page:
Tabletop RPGs are no longer just played in physical spaces. More and more, this kind of game is being played online, and players are discovering online dice rollers, communal game maps, virtual tabletops, and digital character keepers. But too often these tools are walled gardens whose owners see players as products, not human beings.
Togetherness is a web-based virtual tabletop whose chief aim is the empowerment of players as free human beings participating voluntarily in a community. There are no "owners", just equal participants enjoying a gamut of activities including "creation", "play" and "hosting".
The files are all standard SVG format, with JavaScript hooks to provide interactivity where needed. There are live demos, and you can download the code and host your own instance on your own hosting service.