Square DIV where height is equal to viewport

Question

I need to create a DIV where width=height, and height=100% of the viewport (which, obviously, is variable).

In other words, a perfectly sq

Answer 1

This is interesting...

For those who may be looking for a pure CSS way to contain a square div with the maximum dimensions:

The key takeaway here is that you set max-width and max-height while setting width and height to the opposite values.

HTML

CSS

html, body {
  margin: 0;
  padding: 0;
  width: 100vw;
  height: 100vh;
  display: flex;
  flex-direction: row;
  align-items: center;
  justify-content: center;
}

div {
  background-color: red;
  width: 100vh;
  height: 100vw;
  max-width: 100vw;
  max-height: 100vh;
}

Live running example: https://jsfiddle.net/resistdesign/szrv5o19/