How would you rotate a block by 45 degrees?

.block {
  transform: rotate(45deg); /* Rotates the element 45 degrees clockwise */
}

You can also use 3D rotation:

.block {
  transform: rotate3d(0, 0, 1, 45deg); /* Rotates the element around the Z-axis by 45 degrees */
}

For compatibility with older browsers (IE9):

.block {
  -ms-transform: rotate(45deg); /* IE 9 */
  transform: rotate(45deg);
}

For compatibility with other prefixes (-webkit-, -moz-, -o-):

.block {
  -webkit-transform: rotate(45deg); /* Chrome, Safari, Opera */
  -moz-transform: rotate(45deg); /* Firefox */
  -o-transform: rotate(45deg); /* Opera */
  transform: rotate(45deg);
}