When a link within a navigational container points to the current location, it is assigned the .up-current class.
See Navigation bars for more documentation and examples.
Example
Let's look at a simple menu with two links:
<nav> <!-- mark-phrase "up-nav" -->
<a href="/foo">Foo</a>
<a href="/bar">Bar</a>
</nav>
When the browser location changes to /foo, the first link is marked as .up-current:
<nav>
<a href="/foo" class="up-current">Foo</a> <!-- mark-phrase "up-current" -->
<a href="/bar">Bar</a>
</nav>
When the browser location changes to /bar, the first link loses its .up-current class.
Now the second link is marked as .up-current:
<nav>
<a href="/foo">Foo</a>
<a href="/bar" class="up-current">Bar</a> <!-- mark-phrase "up-current" -->
</nav>
