MathML Fundamentals - Error Message merror

The merror element displays its contents as an "error message". This might be done, for example, by displaying the contents in red, flashing the contents, or changing the background color. MathML Fundamentals

Unrecognized element: mfraction; arguments were: 1 + 5 and 2

MathML

<math mathsize="30">
    <merror>
        <mtext>Unrecognized element: mfraction; arguments were:</mtext>
        <mrow>
            <mn>1</mn>
            <mo>+</mo>
            <msqrt>
                <mn>5</mn>
            </msqrt>
        </mrow>
        <mtext>and</mtext>
        <mn>2</mn>
    </merror>
</math>