Re: some bold fonts are badly rendered in snapshot 1.11.901.3 (Official Build) dev (64-bit)
As you can see on the first post Vivaldi.net page has the same problem.
For me the problem has gone, I have upgraded the whole system from Fedora 25 to 26 and now everything seems to look like before yesterday on F25
May be the Linux font substitution used a broken font or font anti-aliasing was not working correctly.
As i said before it was something on your Linux.
Not it works. You should be happy.