A B777 FLT CREW ACCIDENTALLY PASS HOLD SHORT LINE AT SFO.

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AFTER LNDG ON RWY 28L WE EXITED TO TXWY B; AND FOUND OUR GATE OCCUPIED. ATC CLRED US ACROSS RWYS 28L AND 28R TO HOLD SHORT OF RWY 28R ON TXWY S. THEN CLRED 'CROSS RWY 28R; HOLD SHORT OF RWY 28L.' TXWY S DOES NOT CROSS RWY 28L. THERE IS A HOLD SIGN 'ILS RWY 28L' ON TXWY S. WE PASSED THIS SIGN; EXPECTING TO SEE ANOTHER HOLD SIGN PRIOR TO THE EXTENDED EDGE OF RWY 28L. WE DID NOT SEE THIS SIGN AND MAY HAVE PASSED INTO THE CLEARWAY ZONE OF RWY 28L INADVERTENTLY. WE HAD NEVER BEEN ON THIS TXWY AND FOUND THE CTLR'S DIRECTIONS TO BE CONFUSING IN RETROSPECT AS THE TXWY DOES NOT CROSS RWY 28L. ATC DID NOT TELL US WE HAD ERRED. IF THERE IS NO HOLD BAR AND SIGNAGE OTHER THAN THE 'ILS RWY 28R' SIGN THEN THE CTLR SHOULD MAKE THE HOLD SHORT INSTRUCTIONS CRYSTAL CLR. PARTICULARLY SINCE WE TYPICALLY NEVER TAXI ALONG THIS RTE; THE CHANCE OF ERROR IS INCREASED. FOR THIS REASON WE BOTH HAD OUR CHARTS OUT AND HAD ASKED THE CTLR NUMEROUS TIMES DURING OUR TAXI WHERE HE WANTED US TO GO. MANY RWYS HAVE 2 HOLD SHORT LOCATIONS...ONE MARKED 'ILS' AND ANOTHER SIMPLY MARKING THE RWY BOUNDARY. WE EXPECTED TO SEE ANOTHER HOLD POINT PAST 'ILS RWY 28R' SIGN.