fill-opacity

The fill-opacity attribute is a presentation attribute defining the opacity of the paint server (color, gradient, pattern, etc) applied to a shape.

Note: As a presentation attribute fill-opacity can be used as a CSS property.

As a presentation attribute, it can be applied to any element but it has effect only on the following eleven elements: <altGlyph>, <circle>, <ellipse>, <path>, <polygon>, <polyline>, <rect>, <text>, <textPath>, <tref>, and <tspan>

html,body,svg { height:100% }

<svg viewBox="0 0 400 100" xmlns="http://www.w3.org/2000/svg">
  <!-- Default fill opacity: 1 -->
  <circle cx="50" cy="50" r="40" />

  <!-- Fill opacity as a number -->
  <circle cx="150" cy="50" r="40"
          fill-opacity="0.7" />

  <!-- Fill opacity as a percentage -->
  <circle cx="250" cy="50" r="40"
          fill-opacity="50%" />

  <!-- Fill opacity as a CSS property -->
  <circle cx="350" cy="50" r="40"
          style="fill-opacity: .25;" />
</svg>

Usage notes

Note: SVG2 introduces percentage values for fill-opacity, however, it is not widely supported yet (See Browser compatibility below) as a consequence, it is best practices to set opacity with a value in the range [0-1].

Browser compatibility

Specifications