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It could be a good thing to uniformize file XoopsFormRendererLegacy.php (class) with simple quote and double quote for return...

Sometimes, it's (for example):
le="color: #000000"><?php return "<input type='color'...
and some others are:
le="color: #000000"><?php return '<input type="hidden"...
The best way is probably:
le="color: #000000"><?php return '<input type="hidden"...
as HTML principaly uses double quote.