The text-decoration-thickness property controls the thickness of decorations drawn on text — such as underlines, overlines, and line-throughs — independent of the glyph outlines themselves. Rather than changing the stroke of the font, it determines how thick the decorative line that sits above, below, or through the text will be. This lets you make underlines more delicate or more prominent than the browser or font defaults, which is useful for typography refinement, branding, and ensuring visibility against different backgrounds.

In practice, the visual result of text-decoration-thickness depends on font metrics, device pixel density, and the way the user agent renders hairlines and subpixel lines. Thin values may end up as single-pixel hairlines on some screens, while thicker settings scale up and can interact with line gaps and ascenders/descenders of the typeface. When you use larger sizes or apply transforms/zoom, the decoration thickness scales along with the text, but how it relates to stroke-like properties of glyphs varies by rendering engine — so testing across sizes and fonts is recommended to keep decorations from looking disproportionate or from colliding with glyphs.

This property is often used in combination with other decoration-related properties to achieve a complete visual effect: the overall decoration type is still controlled by text-decoration, the vertical placement can be adjusted with text-underline-offset, and the perceived weight of the text itself can influence how thick a decoration should appear (for example, via font-weight). Coordinating these properties lets you create underlines that feel consistent with type weight and style — for instance, slightly thicker underlines for bold text or increased offset when decorations become thick enough to risk touching lowercase letters.

From a practical and accessibility standpoint, use text-decoration-thickness to improve readability and maintain visual hierarchy: subtle underlines for body links, stronger lines for important calls to action, or delicate overlines for stylistic accents. Be mindful of contrast, overlap with diacritics, and responsive behavior — prefer values or strategies that scale with font size so decorations remain proportional across breakpoints and user-scaled text.