Make elements width a percentage based on window

Question

I'm trying to make a max-width of 400px and have the actual width 70% (percentage value). The problem is, the parent element has a value of 6px for its width. Therefore, the child element is 70% of 6px, and I want it to be 70% of the page.

How can I make the childs width a percentage of the full pages width?

In the example below, I'm trying to make .timeline ul li div (line #42), have a max-width: 400px and a width of 70%.

JSFiddle

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  font: normal 16px/1.5"Helvetica Neue", sans-serif;
  background: #456990;
  color: #fff;
  overflow-x: hidden;
  padding-bottom: 50px;
}
/* TIMELINE
–––––––––––––––––––––––––––––––––––––––––––––––––– */

.timeline ul li {
  list-style-type: none;
  position: relative;
  width: 6px;
  margin: 0 auto;
  padding-top: 50px;
  background: #fff;
}
.timeline ul li::after {
  content: '';
  position: absolute;
  left: 50%;
  bottom: 0;
  transform: translateX(-50%);
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background: inherit;
}
.timeline ul li div {
  position: relative;
  bottom: 0;
  /*max-width: 400px;
  width: 70%;*/
  width: 400px;
  padding: 15px;
  background: #F45B69;
}
.timeline ul li div::before {
  content: '';
  position: absolute;
  bottom: 7px;
  width: 0;
  height: 0;
  border-style: solid;
}
.timeline ul li:nth-child(odd) div {
  left: 45px;
}
.timeline ul li:nth-child(odd) div::before {
  left: -15px;
  border-width: 8px 16px 8px 0;
  border-color: transparent #F45B69 transparent transparent;
}
.timeline ul li:nth-child(even) div {
  left: -439px;
}
.timeline ul li:nth-child(even) div::before {
  right: -15px;
  border-width: 8px 0 8px 16px;
  border-color: transparent transparent transparent #F45B69;
}

<section class="timeline">
  <ul>
    <li>
      <div>
        <time>1934</time>At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium
      </div>
    </li>
    <li>
      <div>
        <time>1937</time>Proin quam velit, efficitur vel neque vitae, rhoncus commodo mi. Suspendisse finibus mauris et bibendum molestie. Aenean ex augue, varius et pulvinar in, pretium non nisi.
      </div>
    </li>
    <li>
      <div>
        <time>1940</time>Proin iaculis, nibh eget efficitur varius, libero tellus porta dolor, at pulvinar tortor ex eget ligula. Integer eu dapibus arcu, sit amet sollicitudin eros.
      </div>
    </li>
    <li>
      <div>
        <time>1943</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
    <li>
      <div>
        <time>1946</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
    <li>
      <div>
        <time>1956</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
  </ul>
</section>

Answer 1

Using vw units would work.

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  font: normal 16px/1.5"Helvetica Neue", sans-serif;
  background: #456990;
  color: #fff;
  overflow-x: hidden;
  padding-bottom: 50px;
}
/* TIMELINE
–––––––––––––––––––––––––––––––––––––––––––––––––– */

.timeline ul li {
  list-style-type: none;
  position: relative;
  width: 6px;
  margin: 0 auto;
  padding-top: 50px;
  background: #fff;
}
.timeline ul li::after {
  content: '';
  position: absolute;
  left: 50%;
  bottom: 0;
  transform: translateX(-50%);
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background: inherit;
}
.timeline ul li div {
  position: relative;
  bottom: 0;
  
  width: 70vw;

  padding: 15px;
  background: #F45B69;
}
.timeline ul li div::before {
  content: '';
  position: absolute;
  bottom: 7px;
  width: 0;
  height: 0;
  border-style: solid;
}
.timeline ul li:nth-child(odd) div {
  left: 45px;
}
.timeline ul li:nth-child(odd) div::before {
  left: -15px;
  border-width: 8px 16px 8px 0;
  border-color: transparent #F45B69 transparent transparent;
}
.timeline ul li:nth-child(even) div {
  left: -439px;
}
.timeline ul li:nth-child(even) div::before {
  right: -15px;
  border-width: 8px 0 8px 16px;
  border-color: transparent transparent transparent #F45B69;
}

<section class="timeline">
  <ul>
    <li>
      <div>
        <time>1934</time>At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium
      </div>
    </li>
    <li>
      <div>
        <time>1937</time>Proin quam velit, efficitur vel neque vitae, rhoncus commodo mi. Suspendisse finibus mauris et bibendum molestie. Aenean ex augue, varius et pulvinar in, pretium non nisi.
      </div>
    </li>
    <li>
      <div>
        <time>1940</time>Proin iaculis, nibh eget efficitur varius, libero tellus porta dolor, at pulvinar tortor ex eget ligula. Integer eu dapibus arcu, sit amet sollicitudin eros.
      </div>
    </li>
    <li>
      <div>
        <time>1943</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
    <li>
      <div>
        <time>1946</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
    <li>
      <div>
        <time>1956</time>In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
      </div>
    </li>
  </ul>
</section>

Answer 2

Use viewport units: 70vw

That means 70% of viewport width.

div {

}

.small {
  height: 240px;
  width: 40px;
  background: blue;
 }

.big {
  background: yellow;
  width: 70vw;
  height: 200px;
}

<div class="small">
  <div class="big"></div>
</div>