The html_writer::attribute function is called thousands of times on typical pages (1,271 times on the one I'm testing, to be exact).

This function includes a check, with developer debug warning, for if some idiot passes an array for the attribute value.

Since the check has been there since html_writer was invented (actually before), I think people should really have noticed the developer debug warning now, and this code should be removed.

According to my profiling this check adds ~2ms to a page load on OU systems. Not a lot but a pretty big chunk just for a never-used dev debug warning.

(Note: I'm totally in favour of dev debug checks usually, just not in functions that are called thousands of times on many pages across the system.)