you better use the CSS class rather than the id,

Quite correct.

I'm not completely sure about CSS classes consisting of only numbers being legal

Correct again :-) CSS identifiers (this covers both ID and class names) cannot start with a digit or hyphen. See the spec for the details.

I lied. Underscores have been allowed since the 1998 errata.

As for this idea being expensive at execution time, I hardly doubt so, as the node_id value should be known at page render time anyway for the href attribute

I understood John's suggestion to mean that the class should only be applied to links to home nodes rather than to all links - and he was using the user name rather than the node ID. Knowing the difference between normal nodes and home nodes has got to cost something ;-)