CSS Layout
display:table
Display table was part of the W2C CSS Level 2 spec in 1998
Even though it was part of the W3C spec as early as 1998 it was not supported by the popular browsers of the time until 2008/2009. This meant authors were left using floats if they wanted to use CSS for layout.
It may not have caught on as a way to create page layouts having lost out to floats but it was used as a way to vertically center items.
With the introduction of flexbox vertically aligning items has become much easier.