Is there a way to only use CSS in order to make a button have the same width as a sibling button?

Question

Basically, I have two buttons that have the same style and are lined beside each other inside a parent div.

Answer 1

you could also use flex

div {
  justify-content: center;
  display: flex;
}

.btn {
  flex: 1;
  min-height: 44px;
  width: 100%;
  max-width: 320px;
  position: relative;
  box-sizing: border-box;
  padding: 6px 24px;
  border-radius: 4px;
  outline: none;
  cursor: pointer;
}
.btn.btn-primary {
  border: none;
  background-color: #808080;
  color: #FFFFFF;
}
.btn.button1 {
  background-color: #FFFFFF;
  color: #000000;
  border: 1px solid #b6b6b6;
  flex-shrink: 0 !important;
}
.btn.button2 {
  margin-left: 12px;
  width: auto;
  padding: 6px 36px;
}

  Button1
  Button2

You may also use column CSS

div {
  column-count:2;
  width:max-content;
  margin:auto;
}

.btn {
  min-height: 44px;
  width: 100%;
  max-width: 320px;
  position: relative;
  box-sizing: border-box;
  padding: 6px 24px;
  border-radius: 4px;
  outline: none;
  cursor: pointer;
}
.btn.btn-primary {
  border: none;
  background-color: #808080;
  color: #FFFFFF;
}
.btn.button1 {
  background-color: #FFFFFF;
  color: #000000;
  border: 1px solid #b6b6b6;
  flex-shrink: 0 !important;
}
.btn.button2 {
  margin-left: 12px;
  padding: 6px 36px;
}

  Button1 longer 
  Button2