Set line-height as a percentage relative to the parent element

I have a responsive element where it's width and height will both scale. Inside this I have some text which I want to center vertically.

How can I set the text's line-height to be the same as it's parent if I don't know the parent's height?

line-height: 100% is relative to the font's regular height so this doesn't help...

Here's another way to center an element vertically. I came across this technique some time ago. Basically it uses a pseudo element and vertical-align: middle.

.block::before {
  content: '';
  display: inline-block;
  height: 100%;
  vertical-align: middle;
  margin-right: -0.25em; /* Adjusts for spacing */
}

/* The element to be centered, can
   also be of any width and height */ 
.centered {
  display: inline-block;
  vertical-align: middle;
  width: 300px;
}

Since it's 2019 already, you could also use flexbox to achieve this :)

To do so, add the following classes to the parent element:

display: flex;
flex-direction: column;
justify-content: center;

See this Fiddle

I'd try putting the text inside another element, of which you know (or set) the size. Then setting relative positioning to it, top, left 50% and negative left and right margins.

See this Fiddle

The only problem is that this relies on a known/fixed textblock. If the text is variable, I'm afraid you will have to resort to using Javascript..

Regarding hyperlinks:

I was having this problem regarding links in main menu. And since it was <a> in <li> tags I needed some surface for the links to be clickable/touchable(see touch target size). So what I did was for the <ul> I set a fixed height(through it's parent in this case), the <li>-s are a percentage of it and the <a>-s have a min-height and line-height properties set to them and it's easy from there to set the top. The code:

.menu-header-main-container{
position: fixed;
top: 0;
bottom: 160px;
}    
.menu-header-main-container ul.menu {
          height: 100%; }
          .menu-header-main-container ul.menu li {
            height: 33.33%;
            max-height: 110px; }
            .menu-header-main-container ul.menu li a {
              line-height: 40px;
              min-height: 40px;
              top: calc(50% - 20px);
              position: relative; } }

You cannot set the line-height to 100% of the parent element's height with only CSS. Rather, you can use CSS to center an element vertically.

.parent {
   height:150px;
   position:relative;
   border:1px solid #FDD;
}
.position-center {
    position:absolute;
    top:50%;
    transform:translateY(-50%);
}

<div class="parent">
  <span class="position-center">I am vertically centered element</span>
</div>