If you read the rules then you'd understand why it should not have been a red card.
If you believe that was a deliberate trip, we will ignore the fact he never broke stride, then I think it's you that require an eye sight examination.
https://streamable.com/lw6w1
okay first, he clearly broke stride, look at the replay there, move it to 1/4 speed if you want. Pay attention to the line Varane is running, he's running across him from left to right in the video, and continues in that direction after the contact. But look at his body shape, as he passes Alli, his left leg twists around to make contact, it's actually very deliberate and that replay shows it.
Varane's body and torso should be facing towards the goal line, that is his direction of travel, he slows down and completely twists his torso to bring his left leg across to make contact with Alli. If it was his right leg I could almost believe it was accidental, but precisely because it's the leg on the wrong side and that he had to twist and bring that leg across that makes it deliberate.
It's a correct red card along with a clear as day penalty.