Pixi is setup to render using WebGL, but it has a canvas fallback for older browsers like IE9. If it wasn't working in IE9, it probably wasn't setup correctly or it was using the WebGL renderer.
For Pixi to run, the environment needs to have 3 things available: Object.defineProperty, the Canvas API, and JS image objects. There's polyfills for all those, so you could even get it to work with IE8. Here's a list of supported browsers.
Pixi is a pretty big file. About 75kb gzipped, but it's also on most CDNs. However, you can create a custom build and gut most of that. There's a lot of stuff you probably wouldn't need for doing banner ads.
All I can say is that Pixi is pretty darn awesome. It's used by several game engines like Phaser, and if you look at the GreenSock examples/showcase page you will find that a majority of those projects are also using it.
I don't make banner ads, but to me it seems like it would be a great solution. Instead of dealing with a bunch of SVGs, images, layouts, and fonts that need to be loaded and adapted to different sizes, I would just use Pixi to manage as much as possible. Being able to cache stuff as a bitmap or generating textures on the fly makes it pretty versatile and can handle most things including graphics editing.
For example, the other day I needed some different colored stars, but instead of using a graphics editor I just created them in Pixi. This made it much easier and faster to tweak and fine tune how I wanted them to look, ultimately saving me a bunch of time. From start to finish it took me about 45 minutes to create 5 different spritesheets of stars that go BOOM.