Wikimedia Bugzilla is closed!

mw.html Scribunto library methods such as css, cssText and attr must remove such attribute or property when passed a nil value.

The habit / good pracitce in fluent interface is, that if null is passed, it means that such thing is to be removed.

It allows to not clutter the source code with stuff like

html = mw.html.create( "tag" )
if( classname )
  html:attr( "class", classname )
end

and uses the whole potential of the *fluent* interface thus

html = mw.html.create( "tag" )
  :attr( "class", classname )

which is set if classname is filled or unset(=removed) when it's nil.

Same with css 

html = mw.html.create( "tag" )
  :css( "background", background )

will set the background property if set, or remove from style declaration if nil.

It also allows effective removal of the previously set stuff:

-- default setting
html = mw.html.create( "tag" )
  :css( "background", background )
  :attr( "class", classname )
-- (some code goes here which sets some conditions)
-- based on the condition we decided to remove the preciously set background
if ( condition )
  html:css( "background", nil )
end