Found by SPQRobin.
Notice: Array to string conversion in /home/robin/public_html/mw/dev/includes/Html.php on line 328":
the classes attribute should be passed as a string instead of an array. Introduced in c13632.
Seems to be because Html::dropDefaults() calls strval() on all attribute values, but Html allows arrays as attribute values, which causes a problem. Change to soon be in gerrit.
Version: unspecified
Severity: normal
URL: https://gerrit.wikimedia.org/r/22278
Thanks. I had the impression arrays were not allowed, but they are so your change is a better fix for the problem.
Yeah. Originally I saw the patch and thought to myself "Hmm, why not just accept arrays in Html?", and then I looked and saw it was already accepted. :P
On a side note, I left a comment on https://gerrit.wikimedia.org/r/22005 concerning this, because Hashar is making a whole new test case for the dropDefaults() function, and this would be a perfect addition.