By the way; I like a lot the small animations (rotating legs, rotating weapons), are they precalculated or rendered on the spot?
Drawn right there - they are sprite stacks - literaly sprites drawn on top of each other with each layer shifting upwards 1 pixel at a time.
Found out about it from Project Landsword and Racing Gears Advance.
Legs rotate with the direction of the mech (not the input) which is a simple "currentSpeed + inputDirection * maxSpeed" vector addition (not moveAngle and throttle even though that's how it's controlled). Weapons have an orientation of the "atan2(mousePointInWorld - mechPos)".
If you need to know how exactly everything works, you can read the source in Dev Tools under "index.html" folder - it's not obfuscated. "mech.js" is responsible for drawing the legs, "weapon.js" - the turret and "graphics.js" for pushing tiles to the canvas.
The "spinning hat" of GM and fire flashes of RK are also stacks but rotated around center.
All of that was drawn in MagicaVoxel and exported as "slices".