updated alpha() to make alpha-value required

files/en-us/web/css/reference/values/color_value/alpha/index.md

@@ -25,8 +25,8 @@ alpha(from var(--my-color) / calc(alpha * 0.5))
 - `from <color>`
   - : The [relative color](/en-US/docs/Web/CSS/Guides/Colors/Using_relative_colors) syntax keyword `from` followed by any valid {{cssxref("&lt;color&gt;")}} value, defining the **origin color**. The color components of the origin color are carried through to the result unchanged; only the alpha channel is affected.
 
-- `/ <alpha-value>` {{optional_inline}}
-  - : An {{cssxref("&lt;alpha-value&gt;")}} specifying the alpha of the output color. This can be a `<number>` between `0` and `1`, a `<percentage>` between `0%` and `100%`, or a {{cssxref("calc()")}} expression. Within this value, the keyword **`alpha`** may be used to refer to the alpha channel of the origin color as a `<number>` (where `1.0` is equivalent to `100%`). If this argument is omitted, the alpha of the origin color is used unchanged.
+- `/ <alpha-value>`
+  - : An {{cssxref("&lt;alpha-value&gt;")}} specifying the alpha of the output color. This can be a `<number>` between `0` and `1`, a `<percentage>` between `0%` and `100%`, or a {{cssxref("calc()")}} expression. Within this value, the keyword **`alpha`** may be used to refer to the alpha channel of the origin color as a `<number>` (where `1.0` is equivalent to `100%`).
 
 ### Return value
 
@@ -42,6 +42,10 @@ Unlike general [relative color syntax](/en-US/docs/Web/CSS/Guides/Colors/Using_r
 
 The return value is always in the same color space as the origin color. For example, if the origin color is an [`oklch()`](/en-US/docs/Web/CSS/Reference/Values/color_value/oklch) color, the result will also be resolved in OKLCh, with the same lightness, chroma, and hue.
 
+## Formal syntax
+
+{{CSSSyntax}}
+
 ## Examples
 
 ### Replacing the alpha of a color