The length unit indicates an addition to the default space between words. Values can be negative, but there may be implementation-specific limits. The UA is free to select the exact spacing algorithm. The word spacing may also be influenced by justification (which is a value of the 'text-align' property).

      H1 { word-spacing: 1em }
      

Here, the word-spacing between each word in 'H1' elements would be increased by '1em'.

CSS1 core: UAs may interpret any value of 'word-spacing' as 'normal'.