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):

const style = html.attributes.style;
  const data = style.borderData;
  const children = [];
  const attributes = {
    class: "xfaWrapper",
    style: Object.create(null),
  };
  for (const key of ["top", "left"]) {
    if (style[key] !== undefined) {
      attributes.style[key] = style[key];
    }
  }
  delete style.top;
  delete style.left;
  if (style.position === "absolute") {
    attributes.style.position = "absolute";
  } else {
    attributes.style.position = "relative";
  }
  delete style.position;
  if (style.justifyContent) {
    attributes.style.justifyContent = style.justifyContent;
    delete style.justifyContent;
  }
  if (data) {
    delete style.borderData;
    let insets;
    if (data.margin) {
      insets = data.margin.split(" ");
      delete data.margin;