This is a minimal HTML table for equation formatting. It includes an equation number, but it is not set up to align multiple rows. The style attribute selects a serif font. You should do this with the class attribute instead. The equation itself is a single cell of the table. The equation is centered by two padding columns (each set to width="50%").

(1) left  =  right 
<table border=0 cellspacing=0 cellpadding=0 width="90%" align=center style="font-size: 100%; font-family: times, serif"><tr>
  <td nowrap >(1)</td>
  <td width="50%"></td>
  <td nowrap><i>left</i> &nbsp;=&nbsp; <i>right</i></td>
  <td width="50%"></td>
</tr></table>

This equation includes a fraction as a separate cell.

(2) left  =    top
bottom		   right 
<table border=0 cellspacing=0 cellpadding=0 width="90%" align=center style="font-size: 100%; font-family: times, serif"><tr>
  <td nowrap>(2)</td>
  <td width="50%"></td>
  <td nowrap><i>left</i> &nbsp;=&nbsp; &nbsp;</td>
  <td nowrap align=center><i>top</i><hr noshade size=1><i>bottom</i></td>
  <td nowrap>&nbsp; <i>right</i></td>
  <td width="50%"></td>
</tr></table>

This equation is aligned at the equal sign. All of the lines share a common equation number.

(3) left  
=    top
bottom		   right
 
=  another right 
<table border=0 cellspacing=0 cellpadding=0 width="90%" align=center style="font-size: 100%; font-family: times, serif">
  <tr>
    <td nowrap rowspan=3>(3)</td>
    <td width="50%" rowspan=3></td>
    <td align=right nowrap><i>left</i> &nbsp;</td>
    <td align=left>
      <table border=0 cellspacing=0 cellpadding=0><tr>
        <td nowrap>=&nbsp; &nbsp;</td>
        <td nowrap align=center><i>top</i><hr noshade size=1><i>bottom</i></td>
        <td nowrap>&nbsp; <i>right</i></td>
      </tr></table>
    </td>
    <td width="50%" rowspan=3></td>
  </tr>
  <tr><td>&nbsp;</td></tr>
  <tr>
    <td></td>
    <td align=left nowrap>=&nbsp; <i>another right</i></td>
  </tr>
</table>

This is a fuller version of the previous example. It has fractions on both sides of the equal sign.

(4)
left top
left bottom		    
=    top
bottom		   right
 
=  another right 
<table border=0 cellspacing=0 cellpadding=0 width="90%" align=center style="font-size: 100%; font-family: times, serif">
  <tr>
    <td nowrap rowspan=3>(4)</td>
    <td width="50%" rowspan=3></td>
    <td align=right>
      <table border=0 cellspacing=0 cellpadding=0><tr>
        <td nowrap align=center><i>left top</i><hr noshade size=1><i>left bottom</i></td>
        <td nowrap>&nbsp; &nbsp;</td>
      </tr></table>
    </td>
    <td align=left>
      <table border=0 cellspacing=0 cellpadding=0><tr>
        <td nowrap>=&nbsp; &nbsp;</td>
        <td nowrap align=center><i>top</i><hr noshade size=1><i>bottom</i></td>
        <td nowrap>&nbsp; <i>right</i></td>
      </tr></table>
    </td>
    <td width="50%" rowspan=3></td>
  <tr><td>&nbsp;</td></tr>
  <tr>
    <td></td>
    <td align=left nowrap>=&nbsp; <i>another right</i></td>
  </tr>
</table>

This is a true equation array. Two equations with different equation numbers are aligned.

(5) left   =  right
 
(6) second left   =  much longer second right 
<table border=0 cellspacing=0 cellpadding=0 width="90%" align=center style="font-size: 100%; font-family: times, serif">
  <tr>
    <td nowrap>(5)</td>
    <td width="50%" rowspan=3></td>
    <td align=right nowrap><i>left</i> &nbsp;</td>
    <td align=left nowrap>=&nbsp; <i>right</i></td>
    <td width="50%" rowspan=3></td>
  </tr>
  <tr><td>&nbsp;</td></tr>
  <tr>
    <td nowrap>(6)</td>
    <td align=right nowrap><i>second left</i> &nbsp;</td>
    <td align=left nowrap>=&nbsp; <i>much longer second right</i></td>
  </tr>
</table>

Once the equation is broken up into multiple cells (every example except Equation 1), you have to make sure that the centerline is maintained through the equation. In these examples that happens automatically, but you can get into trouble if different cells have different mixes of subscripts and superscripts, or have different sized fonts. The same thing happens if the numerator of a fraction has a superscript, but the denominator does not. In order to compensate for this, you may need to add empty superscripts or subscripts, to restore the balance. Just one hint:  you can balance a superscript in the numerator with an empty subscript in the denominator. That keeps the denominator closer to the bar than using an empty superscript.