Example
<a-scene>
<a-assets>
<a-mixin id="red" material="color: red"></a-mixin>
<a-mixin id="blue" material="color: blue"></a-mixin>
<a-mixin id="cube" geometry="primitive: box"></a-mixin>
</a-assets>
<a-entity mixin="red cube"></a-entity>
<a-entity mixin="blue cube"></a-entity>
</a-scene>
The entity with red cube will take the properties from the red mixin and the cube mixin in that order. Likewise with the blue cube. Conceptually, the entities above expand to:
<a-entity material="color: red" geometry="primitive: box"></a-entity>
<a-entity material="color: blue" geometry="primitive: box"></a-entity>
