Hover effect : expand bottom border

I'm trying to get a transition hover effect on border that the border expands on hover.

h1 {
  color: #666;
}

h1:after {
  position: absolute;
  left: 10px;
  content: '';
  height: 40px;
  width: 275px;
  border-bottom: solid 3px #019fb6;
  transition: left 250ms ease-in-out, right 250ms ease-in-out;
  opacity: 0;
}

h1:hover:after {
  opacity: 1;
}

<h1>CSS IS AWESOME</h1>

I've tried this on Jsfiddle

Solution 1:

To expand the bottom border on hover, you can use transform:scaleX'(); (mdn reference) and transition it from 0 to 1 on the hover state.

Here is an example of what the border hover effect can look like : Expand border hover effect

The border and transition are set on a pseudo element to prevent transitioning the text and avoid adding markup.
To expand the bottom border from left or right, you can change the transform-origin property to the left or right of the pseudo element:

h1 { color: #666;display:inline-block; margin:0;text-transform:uppercase; }
h1:after {
  display:block;
  content: '';
  border-bottom: solid 3px #019fb6;  
  transform: scaleX(0);  
  transition: transform 250ms ease-in-out;
}
h1:hover:after { transform: scaleX(1); }
h1.fromRight:after{ transform-origin:100% 50%; }
h1.fromLeft:after{  transform-origin:  0% 50%; }

<h1 class="fromCenter">Expand from center</h1><br/>
<h1 class="fromRight">Expand from right</h1><br/>
<h1 class="fromLeft">Expand from left</h1>

Note : You need to add vendor prefixes to maximize browser support (see canIuse).

Expand bottom border on hover with 2 lines

You can achieve this effect when the text spans on 2 lines. The before pseudo element is absolutely positioned to make underline of the first line with bottom:1.2em;:

h1 { position:relative;color: #666;display:inline-block; margin:0;text-transform:uppercase;text-align:center;line-height:1.2em; }
h1:after, h1:before {
  display:block;
  content: '';
  border-bottom: solid 3px #019fb6;  
  transform: scaleX(0);  
  transition: transform 250ms ease-in-out;
}
h1:before{
  position:absolute;
  bottom:1.2em; left:0;
  width:100%;
}
.ef2:hover:after {
  transition-delay:150ms;
}
  
h1:hover:after, h1:hover:before { transform: scaleX(1); }

<h1>Expand border<br/>on two lines</h1>
<br/>
<br/>
<h1 class="ef2">Expand border<br/>effect two</h1>

Different transition direction on hover in and out :

The point is to change the transform-origin position from one side to the other on the hover state. This way the bottom boder enters from one side on hover and exits on the other when the element isn't hovered anymore.
Here is a demo :

h1 { color: #666;display:inline-block; margin:0;text-transform:uppercase; }
h1:after {
  display:block;
  content: '';
  border-bottom: solid 3px #019fb6;  
  transform: scaleX(0);  
  transition: transform 250ms ease-in-out;
}
h1.fromLeft:after{ transform-origin: 100% 50%; }
h1.fromRight:after{  transform-origin:   0% 50%; }
h1.fromLeft:hover:after{ transform: scaleX(1); transform-origin:   0% 50%; }
h1.fromRight:hover:after{ transform: scaleX(1); transform-origin: 100% 50%; }

<h1 class="fromRight">Expand from right</h1><br/>
<h1 class="fromLeft">Expand from left</h1>

Solution 2:

We can do this with only background. No pseudo-element needed. This is more flexible.

h1 {
  /* you can change these variables to control the border */
  --border-color: purple;
  --border-width: 5px;
  --bottom-distance: 0px; /* you can increase this */
  
  color: #666;
  display: inline-block;
  background-image: linear-gradient(var(--border-color), var(--border-color));
  background-size: 0% var(--border-width);
  background-repeat: no-repeat;
  transition: background-size 0.3s;
  margin: 5px 0;
}

.fromCenter {
  background-position: 50% calc(100% - var(--bottom-distance));
}

.fromRight {
  background-position: 100% calc(100% - var(--bottom-distance));
}

.fromLeft {
  background-position: 0 calc(100% - var(--bottom-distance))
}

h1:hover {
  background-size: 100% var(--border-width);
}

<h1 class="fromCenter">Expand from center</h1><br/>
<h1 class="fromRight">Expand from right</h1><br/>
<h1 class="fromLeft">Expand from left</h1>

enter image description here

Multiple line animation:

h1 {
  /* you can change these variables to control the border */
  --border-color: purple;
  --border-width: 5px;
  --bottom-distance: 0px; /* you can increase this */
  
  color: #666;
  display: inline; /* should be 'inline' for multiple line animation */
  background-image: linear-gradient(var(--border-color), var(--border-color));
  background-size: 0% var(--border-width);
  background-repeat: no-repeat;
  transition: background-size 0.5s;
}

.fromCenter {
  background-position: 50% calc(100% - var(--bottom-distance));
}

.fromRight {
  background-position: 100% calc(100% - var(--bottom-distance));
}

.fromLeft {
  background-position: 0 calc(100% - var(--bottom-distance))
}

h1:hover {
  background-size: 100% var(--border-width);
}

<h1 class="fromLeft">Expand from <br>left with <br>multiple line</h1>