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):

mainHtml.children.push(page);
      if (leader) {
        page.children.push(leader[$toHTML](page[$extra].space).html);
        leader = null;
      }
      if (trailer) {
        page.children.push(trailer[$toHTML](page[$extra].space).html);
        trailer = null;
      }
      const contentAreas = pageArea.contentArea.children;
      const htmlContentAreas = page.children.filter(
        node => node.attributes.class === "xfaContentarea"
      );
      for (let i = 0, ii = contentAreas.length; i < ii; i++) {
        const contentArea = (this[$extra].currentContentArea = contentAreas[i]);
        const space = { width: contentArea.w, height: contentArea.h };
        if (leader) {
          htmlContentAreas[i].children.push(leader[$toHTML](space).html);
          leader = null;
        }
        if (trailer) {
          htmlContentAreas[i].children.push(trailer[$toHTML](space).html);
          trailer = null;
        }
        let html = root[$toHTML](space);
        if (html.success) {
          if (html.html) {
            htmlContentAreas[i].children.push(html.html);
          }
          return mainHtml;