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 ui = this.ui ? this.ui[$toHTML]().html : null;
    if (!ui) {
      return HTMLResult.success(html, bbox);
    }
    if (!ui.attributes.style) {
      ui.attributes.style = Object.create(null);
    }
    children.push(ui);
    if (this.value) {
      if (this.ui.imageEdit) {
        ui.children.push(this.value[$toHTML]().html);
      } else if (ui.name !== "button") {
        const value = this.value[$toHTML]().html;
        if (value) {
          if (ui.children[0].name === "textarea") {
            ui.children[0].attributes.textContent = value.value;
          } else {
            ui.children[0].attributes.value = value.value;
          }
        }
      }
    }
    const caption = this.caption ? this.caption[$toHTML]().html : null;
    if (!caption) {
      return HTMLResult.success(html, bbox);
    }
    if (ui.name === "button") {
      ui.attributes.style.background = style.background;
      delete style.background;
      if (caption.name === "div") {