How could it completely compatible with pure css?

[liximomo]

You should take this compatibility seriously, so that we can seamlessly use any available CSS libraries.

[seven-phases-max]

I'm afraid this is too broad request (Please read: https://github.com/necolas/issue-guidelines/blob/master/CONTRIBUTING.md#features referenced in https://github.com/less/less.js#contributing).

There're a just few minor CSS incompatibilities each having its unique reasons (and most of these issues already have their dedicated tickets, e.g. #2207). More over, strictly speaking most of "available CSS libraries" are not even "pure CSS", and the major difference between a compiler and a browser is that the latter can simply ignore incompatible statements while the former can't.

[seven-phases-max]

Closing as too broad (most of CSS incompatibilities already have its dedicated tickets. If you think a specific one is missing - just create that specific use-case ticket).