wide form buttons fixes #3972
auto merge of #4031 : donaldpipowitch/servo/wide-form-buttons, r=jdm

My first pull request to servo \o/

Try to fix https://github.com/servo/servo/issues/3972. Tested with `./mach run tests/html/test-inputs.html`.
Any reasons why this CSS is formatted this way? (All properties on the same line. Looks a little bit _generated_?)