From the beginning of december 17, W3C XHTML started to point out this error:
The above warning is generated inside Joomla script itself to call modules or jQuery, CSS, etc. and is not written in any Joomla file. Probably is related the way Joomla call scripts in core files. This problem also appears in WordPress, but there is a workaround. In Joomla I removed all the modules, the warnings decrease, but still there. I tested with another template, updated Joomla version to 3.8.3 and no solution.