mxGraph is a fully client side JavaScript diagramming library that uses SVG and HTML for rendering. It is the underlying technology that powers the drawing functionality that you see in draw.io. The sources to draw.io are also available.
mxGraph supports IE 9+, Chrome 30+, Firefox 31+, Safari versions actively patched by Apple (6.2.x, 7.1.x, 8.0.x and 9.x at time of writing), Opera 20+, Native Android browser 5.x+, the default browser in the current and previous major iOS versions (e.g. 9.x and 8.x) and Edge 20+.
mxGraph uses no third-party software, it requires no plugins and can be integrated in virtually any framework. Also provided is server-side functionality in Java and .NET for persistence (open and save) functionality, as well as server-side image generation.
In the root folder there is an index.html file that contains links to all resources. You can view the documentation online on the Github pages branch. The key resources are the JavaScript user manual, the JavaScript examples and the JavaScript API specificiation.
There is a mxgraph tag on Stack Overflow that we try to keep answered. Please ensure your questions adhere to the SO guidelines, otherwise it will be closed.