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):

.trim()
      .split(/\s+/)
      .map(x => (x === "-1" ? -1 : getMeasurement(x)));
    this.h = attributes.h ? getMeasurement(attributes.h) : "";
    this.hAlign = getStringOption(attributes.hAlign, [
      "left",
      "center",
      "justify",
      "justifyAll",
      "radix",
      "right",
    ]);
    this.id = attributes.id || "";
    this.layout = getStringOption(attributes.layout, [
      "position",
      "lr-tb",
      "rl-row",
      "rl-tb",
      "row",
      "table",
      "tb",
    ]);
    this.locale = attributes.locale || "";
    this.maxH = getMeasurement(attributes.maxH, "0pt");
    this.maxW = getMeasurement(attributes.maxW, "0pt");
    this.mergeMode = getStringOption(attributes.mergeMode, [
      "consumeData",
      "matchTemplate",
    ]);
    this.minH = getMeasurement(attributes.minH, "0pt");