Gregory's Server
Not so much ongoing, but for fresh development -
The very first thing to test is the Accept: header. I don't know how many times I've encountered an issue with this on new projects.
The second is array and object support - where some popular project only uses a string. You still need to support arrays and objects without crashing.
A lot of people will disagree with me on this one, but the third should be basic HTML support. I don't care if you throw out iframes or font changes, and I can even (barely) tolerate stripping out images and putting them back in the wrong place or order, but you really make the experience horrible and cause a lot of problems for a lot of people if you throw out the blockquote.
The very first thing to test is the Accept: header. I don't know how many times I've encountered an issue with this on new projects.
The second is array and object support - where some popular project only uses a string. You still need to support arrays and objects without crashing.
A lot of people will disagree with me on this one, but the third should be basic HTML support. I don't care if you throw out iframes or font changes, and I can even (barely) tolerate stripping out images and putting them back in the wrong place or order, but you really make the experience horrible and cause a lot of problems for a lot of people if you throw out the blockquote.