[SalesForce] Lightning Web component add an extra DOM element

Building on the answer provided already by @sfdcfox, this following seems to be working for me and allows me to componentize the entire row (much easier to emit events this way).

parent.html

<table class="slds-table slds-table_bordered slds-border_left slds-border_right">
  <thead>
    <tr class="slds-line-height_reset">
      <th class="" scope="col">
        <div class="slds-truncate" title="Name">Name</div>
      </th>
      <th class="" scope="col">
        <div class="slds-truncate" title="Some Custom Column">Some Custom Column</div>
      </th>
    </tr>
  </thead>
  <tbody>
    <template for:each={iterableDataSet} for:item="row">
      <c-child-row row={row}>
      </c-child-row>
    </template>
  </tbody>
</table>

childRow.html

<template>
  <!-- taken straight from SLDS -->
  <th data-label="Name" scope="row">
    <div class="slds-truncate" title={row.Name}>{row.Name}</div>
  </th>
  <td data-label="Some Custom Column">
    <div class="slds-truncate">
      <lightning-input
        data-id={row.Id}
        type="number"
        variant="label-hidden"
        name="inputForDataField"
        onchange={handleOnChange}
        value={row.Data_Field__c}>
      </lightning-input>
    </div>
  </td>
  <!-- more tds .... -->
</template>

And the critical piece on childRow.css

:host {
  display: table-row;
}

Thanks for raising this issue. We recently cleaned up our warning and error messages because some of them were un-actionable or just creating too much noise. This particular warning will no longer be logged in a future release: https://github.com/salesforce/lwc/commit/1afe614236cd0a2c801617afbbdeaeea9b8a30ff