Dave has a feature on Scripting News that he calls tagging, which is basically a reference to all the writing on his site that includes that tag. It’s a similar concept to how Roam, Logseq, and Obsidian use what I call wiki links that not only link to an internal page but also show all places where that page is referenced.
The problem for me is that Dave is using an editing construct in a published form. In my opinion in a published HTML page, a hyperlink is a hyperlink is a hyperlink. Wiki does have the concept of providing an indicator to show a hyperlink is to an external site.
The problem is I find these additional “indicators’ intrusive to reading the page. The underline commonly associated to hyperlinks on web pages is the least intrusive to my reading content.
If it were me, I would keep the appearance of the linsk the same in the HTML, but I would open external links to a new browser tab or page while keeping internal links on the page one is in, if it is even important to the reader to understand the difference between an internal and external link.