Why does ahref tag have an underscore _ ?

Before HTML5, developers used frameset for SPA-like functionality, dividing window into multiple frames, each with its own unique name. for eg, left frame might be name="sidebar" and right could be name="content".

When clicking a link, the browser needed to know which frame to load the content into. That's where the target attribute came in. Clicking a link in sidebar, would load content in content frame:

    <a href="/pricing target="content"></a>

Now if you had a frame named "blank" and used a href="/" target=blank, the content would load in that frame. But if no frame existed, browser would create a new window and assign it the "blank" name.

So Why Underscore ?

It's simple, developers needed a way to explicitly tell the browser to open the link in a new tab, free of frame semantics. The underscore signifies a special value rather than frame name.