What is the colour range of most videos?
Most videos are limited range i.e. luma goes from 16 to 235, and chroma from 16 to 240. 

Unless expressly set, FFmpeg does not scale the range from one to another. Whatever the source is, that's the output.*

However, many inputs don't tag this property in their metadata. So, the output doesn't get tagged either. In that scenario, players tend to assume conventional values. So, a 720p video will get treated as a limited-range BT.709 picture, irrespective of the actual encoding. And some players, especially web ones, don't seem to parse this metadata, and aren't provisioned to adapt to non-default values. So, they will always the treat the input as having conventional encoding.

For safety's sake, HD videos should be encoded to BT.709 Limited range and SD to BT.601 Limited range.

*unless the source only accepts a pixel format whose range is different.