MathML with font-size

math.xml

  <math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
	 <mrow>
	  <mfenced>
	   <mtable>
	    <mtr>
	     
	     <mtd>
	      <mfenced>
	       <mtable>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>1</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>2</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>3</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>4</mn>
	          </msub>
	         </mtd>
	        </mtr>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>5</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>6</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>7</mn>
	          </msub>
	         </mtd>
	         <mtd>
	          <msub>
	           <mi>a</mi>
	           <mn>8</mn>
	          </msub>
	         </mtd>
	        </mtr>
	       </mtable>
	      </mfenced>
	     </mtd>
	     <mtd rowspan="2">
	      <mfenced>
	       <mtable>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>b</mi>
	           <mn>1</mn>
	          </msub>
	         </mtd>
	        </mtr>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>b</mi>
	           <mn>2</mn>
	          </msub>
	         </mtd>
	        </mtr>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>b</mi>
	           <mn>3</mn>
	          </msub>
	         </mtd>
	        </mtr>
	        <mtr>
	         <mtd>
	          <msub>
	           <mi>b</mi>
	           <mn>4</mn>
	          </msub>
	         </mtd>
	        </mtr>
	       </mtable>
	      </mfenced>
	     </mtd>
	    </mtr>
	    <mtr>
	     <mtd>
	      <mtable>
	       <mtr>
	        <mtd>
	         <mspace width="1.25em"></mspace>
	         <mn style="font-size: 1.5em">0</mn>
	         <mi> </mi>
	        </mtd>
	        <mtd>
	         <mfenced>
	          <mtable>
	           <mtr>
	            <mtd>
	             <msub>
	              <mi>c</mi>
	              <mn>1</mn>
	             </msub>
	            </mtd>
	            <mtd>
	             <msub>
	              <mi>c</mi>
	              <mn>2</mn>
	             </msub>
	            </mtd>
	           </mtr>
	           <mtr>
	            <mtd>
	             <msub>
	              <mi>c</mi>
	              <mn>3</mn>
	             </msub>
	            </mtd>
	            <mtd>
	             <msub>
	              <mi>c</mi>
	              <mn>4</mn>
	             </msub>
	            </mtd>
	           </mtr>
	          </mtable>
	         </mfenced>
	        </mtd>
	       </mtr>
	      </mtable>
	     </mtd>
	     
	    </mtr>
	   </mtable>
	  </mfenced>
	 </mrow>
  </math>

This is font-size on a token element with no nodes inside, so that's ok the interpretation of scriptlevel below it does not matter. There is also no font-size: scriptlevel() rules on that element applying AFAIK, but in any case MathML3 / Core says font-size would win and one can always wrap the element into an additional mrow if they want to keep the scriptlevel change.