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):

} else {
      insets = ["0px", "0px", "0px", "0px"];
    }
    let width = "100%";
    let height = width;
    if (insets[1] !== "0px" || insets[3] !== "0px") {
      width = `calc(100% - ${parseInt(insets[1]) + parseInt(insets[3])}px`;
    }
    if (insets[0] !== "0px" || insets[2] !== "0px") {
      height = `calc(100% - ${parseInt(insets[0]) + parseInt(insets[2])}px`;
    }
    const borderStyle = {
      top: insets[0],
      left: insets[3],
      width,
      height,
    };
    for (const [k, v] of Object.entries(data)) {
      borderStyle[k] = v;
    }
    if (style.transform) {
      borderStyle.transform = style.transform;
    }
    const borderDiv = {
      name: "div",
      attributes: {
        class: "xfaBorderDiv",
        style: borderStyle,
      },
    };