What do you think about splitting these docs between render.html and render.json? Docs about POST request will go to render.html, and docs about render.json parameters will go to render.json.

Separate "Javascript Support" section makes sense as a feature documentation, but docs are currently organized around API endpoints, not around splash features. For example, 'wait' parameter documented in render.html endpoint and not in a dedicated section; the note about how is it useful for 'viewport=full' is in render.png. Separate "Javascript Support" section makes other endpoint docs incomplete. It also refers to a not-yet-described 'render.json' endpoint.

Another way is to re-organize docs around features, but currently they are organized around endpoints, and I think it is better to keep them consistent.