Sounded like fun, so I thought I'd try making it.
Here's replacements for two of the files in the "[program files]\Construct 2\exporters\html5\behaviors\sin\" folder.
Set the sin to opacity, magnitude between 0 and 100 (percent). No guarantees, ymmv, glhf, etc.
edit: you might want to back up the original files. (or just reinstall to revert to normal)
edit edit: forgot to specify - the way I implemented, it starts at whatever the initial opacity is set to, and then fades downward as far as [magnitude], and back up to where it started.