According to NFL Network's Ian Rapoport, Williams was absent because he is in search of a new contract. Williams has two years and $24 million remaining on the five-year, $68 million deal he signed in 2015.
Per NFL Network's Tom Pelissero, head coach Jay Gruden addressed Williams' absence and heaped praise on the longtime Redskins star:
The 30-year-old Williams has been the cornerstone of Washington's offensive line since it selected him with the No. 4 overall pick in the 2010 NFL draft out of Oklahoma. Williams is a seven-time Pro Bowler, having participated every year since 2012.
Perhaps the only true concern with Williams is his durability, as he has appeared in all 16 regular-season games just twice during his career, 2012-13. Last season, Williams played in just 13 contests.
Williams has 120 career regular-season games (119 starts) to his credit, though, and there are few players in the NFL who have held down the left tackle position as well as him during that time.
While Williams' contract was significant when he signed it, several offensive tackles have passed him in average annual salary since then. Per Spotrac, Williams ranks sixth in that category among tackles behind Trent Brown, Taylor Lewan, Nate Solder, Jake Matthews and Donovan Smith.
Williams' presence is arguably more important than ever since it is possible that rookie first-round pick Dwayne Haskins could begin the 2019 season as Washington's quarterback.
If Haskins beats out veterans Case Keenum and Colt McCoy for the starting job, he will need reliable blocking on his blind side. Haskins isn't particularly mobile, and he requires time in order to make big throws down the field, which is his biggest weapon.
Provided the Redskins and Williams don't reach an agreement and Williams decided to hold out into the regular season, it would likely press 2018 third-round pick Geron Christian into action at left tackle.