I wonder if my character set or font is wrong in my browser for ancient Greek.
The following quotation is a snip from the Perseus presentation of Smyth. Notice the underline marks, which do not appear in my printed book.
λύ_ω loose, λύ_σω, ἔλυ_σα, λέλυκα, λέλυμαι, ἐλύθην.
(Perseus, Smyth, section 369)
Here is a tinyurl, but you’ll have to scroll down a little: https://tinyurl.com/ywus66s9
The underlines show up in my browser too. I think they’re just a way of showing macrons. Although it’s possible to represent macrons in unicode, I personally find it really hard to read when macrons are superimposed on other stuff like accents and diaresises (diareses?), so I actually like the use of underlines for this purpose. Perseus has a lot of old crufty technology that they haven’t gotten around to changing, such as the use of beta code on some of their web site and old, preserved versions of their github data.
Well, that’s a relief. I agree with you that the underlines are related to macrons. In the example I printed the underlines follow vowels that in my printed copy are marked with macrons.