How can we allow any parent to a specific tag xsd rule

Question

I need to allow any parent tag over <a> tag except <root> tag which means tag have any parent element except root. Example:

 <root>
   <p><a href=""></a></p> //This is correct and valid
</root>

 <root>
   <a href=""></a> //This is inc and valid
</root>

 <root>
   <u><a href=""></a></u> //This is correct and valid
</root>

Allow tag to have any element as parent.

<root><p id="622def0ae96ee826ce70ed87"> 
      <u>   
         <a href="https://offerislive.xyz/offer/eco-fuel-obd2/" rel="nofollow"> 
            <strong>==&gt;&gt;ECO Fuel OBD2 Gasoline Saver Device (Official Website) USA {United States}: Save Fuel Buy 90%::Order At 50% Off Now&lt;&lt;==</strong>    
         </a>   
      </u>  
      <br />    
   </p> </root>

In this above snippet i'm getting validation error as Invalid content was found starting with element 'a'. One of '{p, h2, h3, h4, h5, h6, figure, ul, ol, blockquote, table}' is expected. But i haven't added any rule for this to not allow <u> tag as it's parent. My only validations for u and a tags are:

<xs:element name="u">
        <xs:annotation>
            <xs:documentation>
                underline
            </xs:documentation>
        </xs:annotation>
        <xs:complexType mixed="true">
            <xs:complexContent>
                <xs:extension base="Inline"/>
            </xs:complexContent>
        </xs:complexType>
    </xs:element>
    <xs:element name="a">
        <xs:annotation>
            <xs:documentation>
                content is "Inline" except that anchors shouldn't be nested
            </xs:documentation>
        </xs:annotation>
        <xs:complexType mixed="true">
            <xs:complexContent>
                <xs:extension base="a.content">
                    <xs:attribute name="href" type="xs:string"/>
                    <xs:attribute name="rel" type="LinkTypes"/>
                </xs:extension>
            </xs:complexContent>
        </xs:complexType>
    </xs:element>

Thanks is advance. New bee in xsd.