HTML th tags do not work correctly in the new map viewer, but work fine within the old viewer. Attaching sample code here. The headers all stack at the top of the html, which causes the td to be stacked underneath all of the th tags.
<table style="width: 99%; border-collapse: collapse;">
<tr>
<th colspan="2" style="padding: 8px; text-align: left; border-bottom: 2px solid #555; background-color: #333; color: white; font-weight: bold;">
Group 1 <span style="color: #a2d2ff;">●</span>
</th>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 1</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_1}</td>
</tr>
<tr>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 2</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_2}</td>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 3</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_3}</td>
</tr>
<th colspan="2" style="padding: 8px; text-align: left; border-bottom: 2px solid #555; background-color: #333; color: white; font-weight: bold;">
Group 2 <span style="color: #a689f5;">●</span>
</th>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 1</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_1}</td>
</tr>
<tr>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 2</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_2}</td>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 3</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_3}</td>
</tr>
<th colspan="2" style="padding: 8px; text-align: left; border-bottom: 2px solid #555; background-color: #333; color: white; font-weight: bold;">
Group 3 <span style="color: #a689f5;">●</span>
</th>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 1</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_1}</td>
</tr>
<tr>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 2</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_2}</td>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 3</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_3}</td>
</tr>
<tr>
<th colspan="2" style="padding: 8px; text-align: left; border-bottom: 2px solid #555; background-color: #333; color: white; font-weight: bold;">
Group 4 <span style="color: #fa5050;">●</span>
</th>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 1</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_1}</td>
</tr>
<tr>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 2</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_2}</td>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 3</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_3}</td>
</tr>
<tr>
<th colspan="2" style="padding: 8px; text-align: left; border-bottom: 2px solid #555; background-color: #333; color: white; font-weight: bold;">
Group 5 <span style="color: #b9f5a6;">●</span>
</th>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 1</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_1}</td>
</tr>
<tr>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 2</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_2}</td>
</tr>
<tr style="background-color: #f2f2f2;">
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd; border-right: 1.4px solid #ddd; font-weight: 550;">field 3</td>
<td style="padding: 8px; text-align: left; border-bottom: 1.4px solid #ddd;">{field_3}</td>
</tr> Results:

Results in old map viewer:
