We use WebKitGtk[1] (embedded WebKit) in our presentation software Tech Talk PSE[2]. It would be great if SVG rendering, used for diagrams, was of equal quality to Firefox, but unfortunately Firefox seems to render SVG in a far superior way so we usually have to convert to PNG files to make diagrams embed correctly.
Unfortunately, converting to PNG doesn't work for interactive apps that use SVG as an interactive GUI instead of just a static vector image format. Of course, the user could just install Firefox (or Chrome) to get good-quality SVG support if Apple allowed it, which is why Apple doesn't allow it on iOS. After all, if Apple allowed you to freely choose a fully-working browser, they couldn't force you to use their Apple-only native technologies to get things to work well. It would be much harder to lock you and your customers (sorry, Apple reminds us that they are actually Apple's customers, NOT yours) into Apple's private world.
[1] https://webkitgtk.org/
[2] https://rwmj.wordpress.com/2012/01/31/tech-talk-pse-1-1-0/