California anon voter here:
California Elections Code section 13002
" Ballot paper and ballot cards used by a jurisdiction holding an election pursuant to the laws of California shall be tinted and watermarked or overprinted with a design, to be furnished by the Secretary of State, so that the watermark or overprint is plainly discernible."
SAUCE:
https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=13002.&lawCode=ELEC
SAUCE: (see Article 9)
https://www.sos.ca.gov/administration/regulations/current-regulations/elections/ballot-printing?fbclid=IwAR3NeO2fmz3w40VJs9G0fbULGlkhnKmtPjWEMDGEwK-I56IMibx-0QHEysI#article9
I just checked my California ballot strips and sure enough, there's a watermark.