Print a more helpful debug message when creating an element in the HTML parser.

Using `{:?}` prints a debugging representation such as

```
  collections::string::String{
    vec: collections::vec::Vec<u8>{
      len: 4u,
      cap: 4u,
      ptr: (0x7f75670285d8 as *mut ())
    }
  }
```

which is not very helpful.