The <mphantom> MathML element is rendered invisibly, but dimensions (such as height, width, and baseline position) are still kept.
This element accepts the global MathML attributes.
<math display="block">
<mrow>
<mi>x</mi>
<mo>+</mo>
<mphantom>
<mi>y</mi>
<mo>+</mo>
</mphantom>
<mi>z</mi>
</mrow>
</math>