Links make the web great. Without them, the web will not be any different from plain text files. But some web developers make invalid links. Some of those links do not have the unique identifier that is an underline.
A valid link needs to be underlined. Now I can not say that you must underline all links, because users have gotten used to web patterns. One of the patterns is navigation links. If they do not have an underline, the users will know that they are links. Those links are okay without an underline.
But link text needs an underline. Those links are inside a paragraph. They mingle in a block of text. How do the users differentiate between normal text and link text? The answer is that link text has an underline, while normal text does not have an underline. If you use the HTML u elements (<u>) within a block of text, you should get rid of them. Delete all of them because your users may confuse the underlined text with links.
You may argue, "What if I just give the link text a different color?" The problem with that question is that you do not have enough knowledge about web accessibility. Not all people have the same eyes as you do. There are people who are color blind. Also, Web Content Accessibility Guidelines version 2.2, Success Criterion (SC) 1.4.1 Use of Color, states that we should not use color alone as a way to convey meaning. This means link text must not be displayed through color alone.
Underline your links. You will have valid links.
But, hold on.
I do not say that all underlined text must be links. Your users are smart enough to know when underlined text is not a link. Thus, use your common sense when styling link elements.
There is no simple truth like, "All links must be underlined" or "All links do not need to be underlined anymore." The fact is that some links in a certain context need an underline, while some do not need it. By the way, I am not being politically correct. I am just saying that context matters very much in web development.
To sum up:
- Underline links if they are inside a block of text.
- All underlined text does not need to be links.
- All links do not need to be underlined.
- Context matters very much.
- Your users are not stupid.