Connect multiple elements with single event

Question

Is it possible to connect multiple different elements with a single event on a specific element with only CSS3?

Below is my example case; read the comment in the header selector.

css

#header {
    border-color: darkred;   /* change this along when .nav > li:hover {} */
    border-width: 0 0 3px;
}

.nav > li {
    background-color: darkred;
}

.nav > li:hover {
    background-color: red;
    /*border-bottom: 3px red;*/
}

html

<nav id="header" class="navbar navbar-default navbar-fixed-top">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar"
                    aria-expanded="false" aria-controls="navbar">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Example</a>
        </div>
        <div id="navbar" class="collapse navbar-collapse">
            <ul class="nav navbar-nav">
                <li class="{{ nav_home }}"><a href="#">Home</a></li>
            </ul>
        </div>
    </div>
</nav>

Answer 1

You cannot do this in CSS alone, because it has no ancestor selectors.

However, you can fake the effect by giving #header relative positioning, and adding an empty li at the end of the list, which has the height and width of #header, but with a negative z-index.

Give the empty li these styles:

.nav > li:last-of-type {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  background-color: transparent;
}

And add this style:

.nav > li:hover ~ li:last-of-type {
  background-color: red;
}

Snippet:

#header {
  border-color: darkred;
  border-width: 0 0 3px;
  position: relative;
}

.nav > li {
  background-color: darkred;
}

.nav > li:hover {
  background-color: red;
}

.nav > li:hover ~ li:last-of-type {
  background-color: red;
}

.nav > li:last-of-type {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  background-color: transparent;
}

<nav id="header" class="navbar navbar-default navbar-fixed-top">
  <div class="container">
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand" href="#">Konecranes Trainer</a>
    </div>
    <div id="navbar" class="collapse navbar-collapse">
      <ul class="nav navbar-nav">
        <li class="{{ nav_home }}"><a href="#">Home</a></li>
        <li></li>
      </ul>
    </div>
  </div>
</nav>