Probably leftovers from their templating engine?

I wonder if this really matters with gzip. Or at least whether traffic overhead beats one from implementing and using HTML compression.

I bet they have it measured.

they own so much bandwidth it's probably cheaper for them to just send uncompressed html than to waste time + power recompressing it when things change. Being so large, they're an exception to the general rule.

The more pages you send to a user the costly a single byte on a page is. Conversly the more costly a single byte is the more money you can save by compressing or removing said byte. Owning lots of bandwidth is often highly correlated with sending lots of pages. I think you may find that it's more cost efficient for them to "waste" time and power recompressing than you think.

Scale challenges a lot of your preconceptions when you actually get there.

Mason emits a lot of unnecessary whitespace, but gzip helps a lot.