Just set opacity of the object to value less than 1.0 - see the second text element in the example
(lower one in the figure).
Output
 |
| SVG version |
Source
<svg width="100" height="100" xmlns="http://www.w3.org/2000/svg" >
<g fill="black" font-weight="bold" font-family="sans-serif">
<text x="0" y="40%">Normal text</text>
<text opacity="0.5" x="0" y="60%">Dimmed text</text>
</g>
</svg>