svg – Basic <defs> example

<svg width="400px" height="400px">
<defs>
  <rect id="defrect" fill="blue" fill-opacity=".5" x="50" y="50" width="100" height="100"/>
</defs>

<rect fill="red" x="20" y="20" width="80" height="80"/>
<use xlink:href="#defrect"/>
<use xlink:href="#defrect" x="50" y="60"/>

</svg>

Result

enter image description here

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svg – Basic <defs> example - CodeDay