SVG fill-opacity

The fill-opacity attribute sets the opacity of a shapes fill color. The fill-opacity takes a decimal number between 0 and 1. The lower the value the greater the transparenty is. 

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
    <rect x="30" y="30" height="90" width="170" 
        fill="#529587"
        fill-opacity="0.4"/>
</svg>

Here is a small example:

Text behind shapes