Enables percentage values for the letter-spacing and word-spacing CSS property, as defined in the CSS Text Module Level 4 spec. Percentage values are calculated relative to the advance measure of the space character (U+0020). This gives developers more robust and flexible control over typography, particularly in responsive designs where text spacing needs to adapt to different viewports and font sizes.
The 'letter-spacing' and 'word-spacing' properties currently only accept fixed length values (e.g. px, em), which are inflexible in responsive designs. When font sizes or container dimensions change, this fixed spacing can become disproportionate, forcing developers to write complex media queries or use JavaScript to maintain readable text. Introducing percentage values allows spacing to be defined relative to font-intrinsic metrics. This enables both letter and word spacing to scale proportionally with the font, simplifying the creation of robust and visually consistent typography across all devices without extra code.