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HTML 

              
                
<h1>Columns</h1>

<pre>grid-template-columns: auto 10% auto</pre>

<ol>
  <li>
    <p>While computing the intrinsic width the percentage column is treated as <code>auto</code>.</p>
    <p>The intrinsic width is: 492px.</p>
  </li>
  <li>
    <p>During layout the 10% is resolved to 49px.</p>
    <p>And the track sizing algorithm is run like if you have "<code>grid-template-columns: auto 49px auto</code>".</p>
</ol>

<p>Current output is the same in all browsers (the percentage is treated as <code>auto</code> during intrinsic size computation and resolved during layout):</p>

<div class="grid columns">
  <div class="i1" style="grid-column: 1; grid-row: 1;">Testing</div>
  <div class="i2" style="grid-column: 3; grid-row: 2;">Testing</div>
  <div class="i3" style="grid-column: 1 / span 3; grid-row: 3;">Testing this thing with a quite long line</div>
</div>

<hr>


<h1>Rows</h1>

<pre>grid-template-rows: auto 10% auto</pre>

<ol>
  <li>
    <p>There's no intrinsic height computation phase.</p>
  </li>
  <li>
    <p>During layout the 10% is treated as <code>auto</code>.</p>
    <p>The <em>intrinsic</em> height is 140px. In order to calculate this the track sizing algorithm is already run one time for the rows.</p>
    <p>Then it's supposed that we resolve the 10% percentage to 14px.</p>
    <p>If we want to do that, we need to rerun the algorithm with "<code>auto 14px auto</code>" or we'll have overflow.</p>
</ol>

<p>With just one pass of the track sizing algorithm the output would be something like this:</p>

<div class="grid rows pass-1">
  <div class="i1" style="grid-column: 1; grid-row: 1;">Testing</div>
  <div class="i2" style="grid-column: 2; grid-row: 3;">Testing</div>
  <div class="i3" style="grid-column: 3; grid-row: 1 / span 3;">Testing this thing with a quite long line</div>
</div>

<p>This is because during that 1st pass we got as sizes 20px 100px 20px, and then we resolve the 10% track against 140px (14px). So we resize the 2nd row to 14px and we don't do anything else.</p>

<p>If we do an extra pass we could get the expected output (similar to what we get for columns):</p>

<div class="grid rows pass-2">
  <div class="i1" style="grid-column: 1; grid-row: 1;">Testing</div>
  <div class="i2" style="grid-column: 2; grid-row: 3;">Testing</div>
  <div class="i3" style="grid-column: 3; grid-row: 1 / span 3;">Testing this thing with a quite long line</div>
</div>


<p>Current output is the same in all browsers (the percentage is treated as <code>auto</code> and never resolved later):</p>

<div class="grid rows">
  <div class="i1" style="grid-column: 1; grid-row: 1;">Testing</div>
  <div class="i2" style="grid-column: 2; grid-row: 3;">Testing</div>
  <div class="i3" style="grid-column: 3; grid-row: 1 / span 3;">Testing this thing with a quite long line</div>
</div>
!

CSS 

              
                
.grid {
  font: 20px/1 Monospace;
  display: inline-grid;
  border: solid thick;
}

.columns {
  grid-template-columns: auto 10% auto;
  grid-template-rows: 50px 50px 50px;
}

.rows {
  grid-template-columns: 100px 100px 100px;
  grid-template-rows: auto 10% auto;
}

.pass-1 {
  grid-template-rows: 20px 14px 20px;
  margin-bottom: 80px;
}

.pass-2 {
  grid-template-rows: auto 14px auto;
}

.i1 {
  background: magenta;
}

.i2 {
  background: cyan;
}

.i3 {
  background: yellow;
}
!

JS
!
999px

Console