Reading xhtml file with Nokogiri give tag invalid errors

I am using rails 5 with ruby 2.3
I am using gem nokogiri (1.8.5) to read my xml, svg and xhtml files.

While reading my xhtml file the nokogiri gives me following errors.

> [#<Nokogiri::XML::SyntaxError: 8:159: ERROR: Tag x3d invalid>,
> #<Nokogiri::XML::SyntaxError: 9:21: ERROR: Tag scene invalid>, #<Nokogiri::XML::SyntaxError: 10:37: ERROR: Tag navigationinfo invalid>, #<Nokogiri::XML::SyntaxError: 11:45: ERROR: Tag background
> invalid>, #<Nokogiri::XML::SyntaxError: 14:47: ERROR: Tag viewpoint
> invalid>, #<Nokogiri::XML::SyntaxError: 15:75: ERROR: Tag transform
> invalid>, #<Nokogiri::XML::SyntaxError: 16:69: ERROR: Tag shape
> invalid>, #<Nokogiri::XML::SyntaxError: 17:55: ERROR: Tag appearance
> invalid>, #<Nokogiri::XML::SyntaxError: 19:81: ERROR: Tag
> commonsurfaceshader invalid>]

is there any way to add custom tags to nokogiri, so that I can parse my xhtml and fetch the useful data from it.
In my xhtml I am using x3d tags also
Thanks in advance