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):

"xfa-font-vertical-scale",
  "xfa-spacerun",
  "xfa-tab-stops",
]);
const StyleMapping = new Map([
  ["page-break-after", "breakAfter"],
  ["page-break-before", "breakBefore"],
  ["page-break-inside", "breakInside"],
  ["kerning-mode", value => (value === "none" ? "none" : "normal")],
  [
    "xfa-font-horizontal-scale",
    value =>
      `scaleX(${Math.max(0, Math.min(parseInt(value) / 100)).toFixed(2)})`,
  ],
  [
    "xfa-font-vertical-scale",
    value =>
      `scaleY(${Math.max(0, Math.min(parseInt(value) / 100)).toFixed(2)})`,
  ],
  ["xfa-spacerun", ""],
  ["xfa-tab-stops", ""],
  ["font-size", value => measureToString(1 * getMeasurement(value))],
  ["letter-spacing", value => measureToString(getMeasurement(value))],
  ["line-height", value => measureToString(0.99 * getMeasurement(value))],
  ["margin", value => measureToString(getMeasurement(value))],
  ["margin-bottom", value => measureToString(getMeasurement(value))],
  ["margin-left", value => measureToString(getMeasurement(value))],
  ["margin-right", value => measureToString(getMeasurement(value))],
  ["margin-top", value => measureToString(getMeasurement(value))],
  ["text-indent", value => measureToString(getMeasurement(value))],