com.raquo.domtestutils.codecs

This trait represents a way to encode and decode HTML attribute or DOM property values.

It is needed because attributes encode all values as strings regardless of their type, and then there are also multiple ways to encode e.g. boolean values. Some attributes encode those as "true" / "false" strings, others as presence or absence of the element, and yet others use "yes" / "no" or "on" / "off" strings, and properties encode booleans as actual booleans.

Scala DOM Types hides all this mess from you using codecs. All those pseudo-boolean attributes would be simply Attr[Boolean](name, codec) in your code.