An alpha channel controls how transparent or opaque a color appears. A value of 1.0 makes text completely opaque; 0.0 makes it completely transparent. section 17.4
Transparency lets whatever sits behind text show through, whether that is a solid color, an image, or other text. Use it sparingly when readability matters. section 17.4
CSS provides several color formats that can carry an alpha value:
- Use rgba() or hsla() with a fourth parameter for the alpha channel. section 17.4
- Use an eight-digit hexadecimal color in the form #rrggbbaa; the final aa pair controls transparency. section 17.5
- In hexadecimal notation, 00 makes the color completely transparent and ff makes it completely opaque. section 17.5
A transparent purple heading
CSS color functions accept the alpha value after the color components: hsl(300, 100%, 25%, 0.5) makes the purple heading partly transparent. The equivalent hexadecimal form places the alpha pair last, as in #80008080. section 17.4 section 17.5
