"The most beautiful crab spider in California" according to Robert Z. Schick, author of 1965 "The Crab Spiders of California".

At the time of writing in 1965, this was called Diaea pictilis...a species described in 1896 and considered native to coastal California from Sonoma & Napa counties south to San Diego and into northern Baja California.

Only later was it synonymized with D. livens...a species of central & southern Europe and reaching east to Iran. Presuming that synonymy is sound, it suggests this may be an introduced species in CA.

For more details on the ID here (with lots of supporting links & references) see this companion BugGuide post.