The atan(number) function accepts only one value as its parameter.

number

A calculation which resolves to a <number> between -∞ and +∞.

The inverse tangent of an number will always return an <angle> between -90deg and 90deg.

• If number is 0⁻, the result is 0⁻.
• If number is +∞ the result is 90deg.
• If number is -∞ the result is -90deg.

That is:

• atan(-infinity) representing -90deg.
• atan(-1) representing -45deg
• atan(0) representing 0deg
• atan(1) representing 45deg
• atan(infinity) representing 90deg.

<atan()> = 
  atan( <calc-sum> )  

<calc-sum> = 
  <calc-product> [ [ '+' | '-' ] <calc-product> ]*  

<calc-product> = 
  <calc-value> [ [ '*' | '/' ] <calc-value> ]*  

<calc-value> = 
  <number>         |
  <dimension>      |
  <percentage>     |
  <calc-constant>  |
  ( <calc-sum> )   

<calc-constant> = 
  e          |
  pi         |
  infinity   |
  -infinity  |
  NaN        

The atan() function can be used to rotate elements as it return an <angle>.

HTML

<div class="box box-1"></div>
<div class="box box-2"></div>
<div class="box box-3"></div>
<div class="box box-4"></div>
<div class="box box-5"></div>

CSS

div.box {
  width: 100px;
  height: 100px;
  background: linear-gradient(orange, red);
}
div.box-1 {
  transform: rotate(atan(-99999));
}
div.box-2 {
  transform: rotate(atan(-1));
}
div.box-3 {
  transform: rotate(atan(0));
}
div.box-4 {
  transform: rotate(atan(1));
}
div.box-5 {
  transform: rotate(atan(99999));
}

Result

• sin()
• cos()
• tan()
• asin()
• acos()
• atan2()