How do Font-Size, Line-Height, and Actual Height Interact to Determine Element Height?

Font-Size vs Line-Height vs Actual Height: Demystified

The interplay between font-size, line-height, and actual element height can be puzzling. Let's delve into these concepts and their impact on web element display.

Font-Size: The Container for Characters

Font-size defines the height of a rectangular box that contains the characters (letter forms) of an inline element. This height encompasses both ascenders (strokes that extend above the main body of the text) and descenders (those that extend below it).

Line-Height: The Box Containing All Lines

Line-height specifies the height of the line box, which encapsulates all the lines within an inline element. In simple cases, it's equivalent to the font-size of the element's largest line.

Height Determinations

The height of an inline element is determined by its line-height, while the height of its parent block element is influenced by the line-height of both the inline child and the block parent.

• Parent blocks establish inline formatting contexts with height based on the last line box.
• Within the parent block, the line-height of the inline element acts as a "strut," setting a minimum height above and below the baseline. This affects the parent block's height.

Actual Height: Beyond Font-Size and Line-Height

The actual height of the content area within an inline element, such as text painted by a background color, is not directly derived from font-size or line-height. This height can vary depending on the font metrics and rendering implementation.

Line-Height and Parent Block Height

Setting the parent block's line-height to 0 effectively ignores the inline element's line-height, allowing the parent block's height to match the inline element's line-height (but not necessarily its font-size).

Unexpected Results

When an inline element's line-height is less than its font-size, the actual height of the element (its contents) can be smaller than the line box height. Conversely, when the line-height is greater than the font-size, the actual height can exceed the line box height. These deviations are due to the strut's minimum height above and below the baseline.

Practical Implications

Understanding these concepts helps predict element height behavior and fine-tune layouts. For example, setting the line-height of a parent block to the same value as the font-size of its inline child elements ensures that the block element's height will match their line height.

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