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 children = [];
    this[$extra].children = children;
    const style = Object.create(null);
    if (this.medium && this.medium.short && this.medium.long) {
      style.width = measureToString(this.medium.short);
      style.height = measureToString(this.medium.long);
      if (this.medium.orientation === "landscape") {
        const x = style.width;
        style.width = style.height;
        style.height = x;
      }
    } else {
      warn("XFA - No medium specified in pageArea: please file a bug.");
    }
    this[$childrenToHTML]({
      filter: new Set(["area", "draw", "field", "subform", "contentArea"]),
      include: true,
    });
    return HTMLResult.success({
      name: "div",
      children,
      attributes: {
        id: this[$uid],
        style,
      },
    });
  }
}
class PageSet extends XFAObject {