In an initiative proposed by Milan - and followed by five other Serie A clubs - the players wore jerseys featuring maternal surnames. Pulisic had 'Harlow.'
Second-placed Milan remained seven points above Bologna, who moved into third after winning at Napoli 2-0, and eight points above Juventus ahead of the Bianconeri's match against already-relegated Salernitana on Sunday.
Cagliari remained three points above the drop zone.
Milan were looking to snap a six-match winless run, and broke the deadlock 10 minutes from time when the ball bounced around the area to an unmarked Ismael Bennacer, who volleyed his team into the lead.
Leao hit the crossbar in the second half before setting up Pulisic in the 59th minute to double Milan's lead.
Nahitan Nández pulled one back for Cagliari four minutes later but Milan restored their two-goal advantage in the 74th with a 30-yard rocket from Tijjani Reijnders.
Cagliari came close to reducing the deficit again 10 minutes from time when Gaetano Oristanio's in-swinging corner came off the right post and back out to Eldor Shomurodov, who then hit the crossbar.
Milan scored twice more through Leao and Pulisic, whose finish was not cleared off the line in time by Yerry Mina.
Last season's champions, eighth-placed Napoli were heavily booed during the home loss to Bologna.
A drama-filled opening 20 minutes saw Napoli concede two goals and have a penalty saved.
Dan Ndoye and Stefan Posch scored headers in a three-minute span for Bologna. That sparked resounding jeers from the home support.
Napoli received a penalty in the 20th minute after Victor Osimhen was fouled by Remo Freuler but Matteo Politano's spot kick was parried by Federico Ravaglia.
8°C