htmlgen: what am I doing wrong? "cannot extract identifier from node: class" error

[goldstein]

This code: [https://pastebin.com/J9sL0ZsF](https://pastebin.com/J9sL0ZsF) 
(where `hg` is `htmlgen`) results in this error: 
[https://pastebin.com/Ti8qkWvg](https://pastebin.com/Ti8qkWvg) on `hg.h3` line. 
I can't understand what exactly am I doing wrong.