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 attributes = {
      style,
      id: this[$uid],
      class: classNames.join(" "),
    };
    if (this.name) {
      attributes.xfaName = this.name;
    }
    const html = {
      name: "div",
      attributes,
      children: [],
    };
    const extra = addExtraDivForBorder(html);
    const bbox = computeBbox(this, html, availableSpace);
    const value = this.value ? this.value[$toHTML](availableSpace).html : null;
    if (value === null) {
      return HTMLResult.success(extra, bbox);
    }
    html.children.push(value);
    if (value.attributes.class === "xfaRich") {
      if (this.h === "") {
        style.height = "auto";
      }
      if (this.w === "") {
        style.width = "auto";
      }
      if (this.para) {