Hailee Steinfeld Stuns in Leggy Dress During Wedding Rehearsal with Josh Allen

Hailee Steinfeld's practicing with Josh Allen ... no, they're not running routes on the gridiron -- they're rehearsing for their luxurious wedding!

The actress/singer and her footballin' fiancé were spotted standing on the altar together Friday ... Hailee wearing a short white dress and flashing a huge grin for the assembled crowd.

Allen's not taking the chance of getting sunburnt in the California heat ... holding up an umbrella to keep from burning up out there.

Steinfeld got flirty with her man in the shade ... laughing and leaning in for a smooch -- and, it's easy to see that these two are super ready to make a lifelong commitment to one another.

Hailee was also spotted chatting with some pals at the fabulous California resort where the wedding's supposed to go down.

It looks like she's making sure everyone knows she's the bride all weekend, too -- sticking to white even before she dons her wedding dress.

Rumors of a relationship between Hailee and Josh exploded a little more than 2 years ago ... when the two looked loved up inside a sushi restaurant.

Just weeks later, the pair confirmed their romantic entanglement ... when Hailee was fully on Josh's lap in a hot tub -- blessing fans with a look at their even hotter make-out sesh.

While they've stayed out of the public eye -- at least a bit more than another entertainer/NFLer couple we know -- a gender reveal video featuring the two showed how smitten with one another they are months after they started dating.

And smitten they were ... 'cause Allen got down on one knee and asked Steinfeld to marry him just a few months later.

Unclear when exactly the wedding's going down ... or where the two are honeymooning -- though they've got a lot of time before Allen's expected back in Buffalo for training camp.