English teacher here. Articles in English can be really confusing but essentially we use the definite article in this situation because:
- Uniqueness: In most situations, there's only one mirror in a room or a home that's readily available for someone to look into.
- Generality: Similar to "going to the bathroom," "look in the mirror" refers to the general act of using a mirror to see oneself, not interacting with any specific mirror.
The format actually has a lot of benefits - it supports transparency, animation, and compresses very efficiently. So it could theoretically replace GIF, JPG, and PNG in one fell swoop.
The downsides are that many apps don't currently support it and that it's owned by Google.
Personally I use webp for images that are not intended to share (e.g. banners and images on my blog), but stick to JPG/PNG for sending to other people.