Repro steps:
1) Open the project and Main.scene
2) Build and run it for WebGL
3) In the browser open up the Console
4) Notice the difference in the "Imported clip length" and the "WWW loaded clip state: Loaded length" (look at screenshot)

N.B. It seems the shown WWW length is original length multiplied by ~4.35

Expected: both values should be the same

Reproduced in: 5.4.0b13; 5.3.4p1 || Windows 10
Different issue in 5.2, cannot test

No Repro on OSX