Later - looks like it is only some accounts affected, I just logged into another and it seems to be OK for now. But using the basic link above, then selecting to "always open in basic HTML" will get rid of the pesky new styles when they appear. After that you just log on normally, using Basic HTML is saved as your preference.
The things you lose are
Chat - neither want nor need
Spell checker - occasionally useful, which is why I use a browser (Opera) that has one built in. Are there any that don't these days?
Keyboard shortcuts - not sure which ones they mean, the usual copy, paste etc. still work.
Ability to manage filters - rarely use
Adding, editing, importing, or deleting contacts - rarely use
Custom 'From:' addresses - don't use
Rich formatting - don't use