There’s no question Valve made some of the best games of the previous generation and four of those have joined the Enhanced Xbox Backward Compatibility program.
UPDATE (10/23/2018): It seems these games were already part of the Backward Compatibility program and are joining the Enhanced Xbox 360 Backward Compatibility program meaning they are enhanced for the Xbox One X.
ORIGINAL STORY (with modifications):
The games joining the Enhanced program include Left 4 Dead, Left 4 Dead 2, Portal: Still Alive, and Half-Life 2: The Orange Box. The Orange Box is a fantastic deal in itself with five games: Half-Life 2, Half-Life 2: Episode One, Half-Life 2: Episode Two, Portal, and Team Fortress 2.
Xbox 360 games enhanced for Xbox One X run at a higher resolution and 9X the original pixel count. The power of Xbox One X enables the Xbox 360 emulator to showcase the very best version possible with the existing assets—all without touching the game code. Today’s additions join the 17 previously released enhanced Xbox 360 titles like Red Dead Redemption and Skate 3 for a total of 21 Xbox One X Enhanced Xbox 360 games.
The four new additions to the program bring the Enhanced Xbox 260 Backward Compatibility totals to 21 titles, joining over 500 Xbox 360 games, and 32 Original Xbox games.Source: Xbox Wire