Function parse_line_height_number 

fn parse_line_height_number(
    context: &ParserContext<'_>,
    input: &mut Parser<'_>,
) -> Result<NonNegativeNumber, ParseError>

Parses a line height value. Percentages in -typed calc expressions are allowed in the line-height property, relative to the computed value of 1em. https://drafts.csswg.org/css-inline/#line-height-property