Using AE for web animation/ads?

Kyle is right, the Bodymovin script for Ae is amazing and will support lots of vector based animation. It can be output to a json file. thoroughly recommend looking at it as it allows you to stay in AE. You can also output GIfs again now from AE so all in all it can be done. Animate is a great app but a whole different system from AE.

Really there also a cool tools from Airbnb it can help doing some animation and used in website or mobile application sure after converted to JSON

Check it here Lottie – Airbnb Design

AE makes video. There are some ways to get animated gif's and even very limited support for SVG but for professional work AE is the wrong tool. Sure, you can kind of get by using a tool not designed at all for interactivity or direct to web graphics, but if you want to get more than minimum wage for your time, take the time to learn Animate so you can produce something truly effective that clients won't have to fiddle with for hours to make work.

While I don't have tons of experience with it myself, it IS possible to export AE animations for the web, with some limitations and caveats.

This tool will help: Bodymovin - aescripts + aeplugins - aescripts.com
As will this: Lottie