Please specify the actions an user-agent should do with invalid values

Hello,

   Please specify the actions an user-agent should do with invalid values:

* ignore the deepest tag with the invalid value?
* ignore the invalid value and use the default value for the attribute in  
the tag?
* ignore the invalid value and use the default values for all the  
attributes in the tag?
* use the nearest valid value for the invalid value?


For example,
http://www.w3.org/html/wg/html5/#font


<style>
   blockquote mark, q mark {
     font: inherit; font-size: 0px;
                               ^^^^
   }
</style>

or using the deprecated sample:

  <font size=0>sample text</font>


Thanks in advance.
Best Regards.