1. Add the HTML field in your report template with safe rendering:

​<t t-raw="object.html_field"/>

2. Define your own <style> block inside the report QWeb template

• Add basic table styling inline in your template, like this:

​<t t-name="your_module.report_template_name">

​  <t t-call="web.html_container">

​    <t t-set="doc" t-value="doc"/>

​

​    <style type="text/css">

​      table {

​        width: 100%;

​        border-collapse: collapse;

​      }

​      th, td {

​        border: 1px solid black;

​        padding: 5px;

​        text-align: left;

​        vertical-align: top;

​      }

Important Notes

• Avoid t-call-assets inside reports; they are meant for HTML web views, not PDF reports.
• Even if you're printing reports in HTML (not PDF), the safest route is still to define the CSS directly.
• If your HTML content uses classes like .row or .col-6 (like Bootstrap), you need to manually define those classes in your <style> section, because they are not included by default.

Thanks & Regards, 

Company : DataInteger Consultancy Services LLP