This specification provides features to align lines and blocks to invisible grids in the document.
Aligning lines and blocks to grids provides the following benefits:
Vertical rhythm is kept for better readability.
Lines are aligned between columns in multi-column documents.
The top and the bottom margins of pictures are made equal, while keeping the vertical rhythm of text before and after the pictures.
Layout lines are at the same position on every page in paged media. Keeping the position of the bottom line of a page has benefits for design and readability. This also improves the readability of duplex printing, two pages spreads, and displaying on slow display devices like e-ink.
East Asian layouts require vertical rhythm more often than other scripts do, even in single column, non-paged media documents, as defined in [JLREQ].