Scalable Widths: em and percentage units on text columns.

This column’s width is set in em units and scales with the font-size.

This is an accessible method for approximating character length on columns because very wide lines of text become difficult to read.

This column’s width is set as a percentage of the page width and scales with the window size.

This is great for a flexible layout, but consider using min-width and max-width values in em sizes.