This is the complete list of tags that will render as HTML when defined as placeholder text:
Hyperlinks: <A HREF>
Fonts: <FONT>
Header, style and block elements: <H{n}>, <DIV>, <SPAN>,<P>, <DIV>, <LI>, <HN>
Text format: <B>, <I>, <U>, <S>
List handling: <OL>, <UL>, <LI>
So, when importing HTML content into a report, we need to pay attention to the supported tags. If the HTML content includes HTML tags that are not supported, the HTML that is shown in the rendered report may differ from the original HTML.
And about the thing
<body style="overflow:hidden">
It's a bit complex one. You have to edit in your ReportManager CSS, not in report level expression.