`normal` to `running`

[svgeesus]

From 2. Controlling Image Animations: the image-animation property

running: the image animation plays, without synchronization with any other instance of the same image on the page.

If the value is changed from normal to running, does it:

• keep running from where it was, but now with no guarantee of future synchronization
• start running from the start of the animation

[DevSDK]

This was also questioned by a prototype CL reviewer on Chromium.

FYR: https://chromium-review.googlesource.com/c/chromium/src/+/7458159/comment/33801773_d3ff9ea5/