[VarDumper] Improved dump in html

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | http://www.w3.org/TR/html5/dom.html#the-id-attribute
| License       | MIT
| Doc PR        | -