What is "Toxi"? Where did that come from?
Wherever, it's also the properly normalized version.
If you truly have the limitations you specified and you 100%-promise-to-delete-the-entire-site-if-you-change-your-mind-kind-of-sincerity that those limitations won't changed, then a little bit of denormalization as you propose would surely not hurt.
Would it help? I tend to doubt it.
I would like to point out that if you do denormalize then there are several possible future queries that get tougher. Example:
"How many bookmarks use both tag13 and tag93?"
[WHERE (tag1=13 AND tag2=93) OR (tag1=93 AND tag2=13)]
"How many bookmarks have only one tag?"
[WHERE (tag1 IS NOT NULL AND tag2 IS NULL) OR (tag1 IS NULL AND tag2 IS NOT NULL]
"How many bookmarks use exactly the same two tags as another bookmark?"
[much more code than I want to show]
All those--and more--are trivial in the fully normalized ("Toxi") case.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.