Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Firefox 88.0.1
- Operating system and its version: Windows 10.0.19043.985]
- PDF.js version: (commit 706c27b9d4b3b9ff6fc3de82bcf71a0610604a1c) (also tested pr/13473 at 296bbdc4)
- Is a browser extension: N/A

Steps to reproduce the problem:
1. Enable experimental XFA
2. Open PDF
[dir2020-02_form.pdf](https://github.com/mozilla/pdf.js/files/6571084/dir2020-02_form.pdf)

What is the expected behavior? (add screenshot)
Margins of `subforms` (e.g. *General_Election_Information*, *General_Information_Other*) are honored.
![image](https://user-images.githubusercontent.com/7104454/120212605-32066300-c200-11eb-9c66-f2d05c9451ec.png)
What went wrong? (add screenshot)
Margins are not honored. 
![image](https://user-images.githubusercontent.com/7104454/120212535-1ac77580-c200-11eb-8332-9924602ef991.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

import { getMeasurement, HTMLResult } from "./utils.js";
const XHTML_NS_ID = NamespaceIds.xhtml.id;
const VALID_STYLES = new Set([
  "color",
  "font",
  "font-family",
  "font-size",
  "font-stretch",
  "font-style",
  "font-weight",
  "margin",
  "margin-bottom",
  "margin-left",
  "margin-right",
  "margin-top",
  "letter-spacing",
  "line-height",
  "orphans",
  "page-break-after",
  "page-break-before",
  "page-break-inside",
  "tab-interval",
  "tab-stop",
  "text-align",
  "text-decoration",
  "text-indent",
  "vertical-align",
  "widows",
  "kerning-mode",
  "xfa-font-horizontal-scale",