Usually when I'm writing slowly and carefully I can make it quite clean (not at level of this video but close enough for an amateur that didn'teven knew hiragana last year) but when I'm actually writing fast? I'm probably creating a new alphabet at that point :))
I don't like how "fu" is looking at me... Jokes aside, this is appreciated. It's hard to tell from an outsider perspective which parts of a written language are considered more critical to its legibility by native users.
Better answer: Usually, only computers tend to leave that line filled, in writing i highly recommend you don't fill the line. I used to and it looked really awkward, and additionally it requires one less stroke to fill it so you're arguably not doing it properly. Just if you want your hiragana to look good, don't fill the lines, but there are probably japanese people out there who do fill it when they write since natives kinda get an excuse to
