mootools/mootools-core — comment on commit ef71402244944f512645266c826a7af74c3d9b6a
File: Source/Element/Element.Style.js line 49
By jbgithub:

Also you may think of changing

if (opacity == null || opacity == 1) opacity = '';

to

if (opacity == null || opacity >= 1) opacity = '';

but it would not catch values that are rounded to 100.