Do Specific Text Characters Actually Affect Line Height in CSS?

Line Height Alterations by Specific Text Characters

Overview

This question investigates the phenomenon of specific text characters potentially altering the line height of an element. Initially, we'll examine the provided code snippet and observe the unexpected behavior exhibited in certain browsers.

Code Snippet Analysis

The following code snippet illustrates the issue:

<p></p>

The purpose of this code is to set the line height to 1 and hide any text that overflows the available space. When rendered in Firefox on Windows 10, however, the behavior deviates from expectations. The underscore character ("_") is rendered without an underscore, and the subsequent character is cut off. Other browsers, such as Chrome, render the underscore normally.

Underlying Factors

Font Differences:
All browsers render text using different default fonts, which can affect the line height. The line-height property specifies a multiplier for the font's size, and different fonts can have varying heights for the same font size.

Character Extents:
Characters can extend beyond the x-height (the height of lowercase letters) of the font. This is more pronounced in characters like "p," "g," and "j," which extend below their x-height. If the line box height is set to 1x font-size, characters that extend below or above this height may overflow.

Correct Behavior

Both behaviors observed in different browsers can be considered correct, as they adhere to the specified line height and the font's characteristics. The line box height is equal to the font size multiplied by the line-height value (1 in this case), and the font determines the character extents.

Character Effect on Line Height

Characters cannot directly change the line height. The line height property controls the size of the line box, but the content area within the line box is determined by the font's design. Characters that extend beyond the x-height can cause the content to overflow the line box, which may result in clipping or line height stretching.

Conclusion

The line height uncertainty originates from the combination of default font differences and character extents. Setting line-height: 1 does not guarantee that any text will fit exactly within the line box, as it depends on the font's specific design.

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