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):

breakingNode: null,
      pageNumber: 1,
      pagePosition: "first",
      oddOrEven: "odd",
      blankOrNotBlank: "nonBlank",
    };
    const root = this.subform.children[0];
    const pageAreas = root.pageSet.pageArea.children;
    const mainHtml = {
      name: "div",
      children: [],
    };
    let pageArea = null;
    let breakBefore = null;
    let breakBeforeTarget = null;
    if (root.breakBefore.children.length >= 1) {
      breakBefore = root.breakBefore.children[0];
      breakBeforeTarget = breakBefore.target;
    } else if (
      root.subform.children.length >= 1 &&
      root.subform.children[0].breakBefore.children.length >= 1
    ) {
      breakBefore = root.subform.children[0].breakBefore.children[0];
      breakBeforeTarget = breakBefore.target;
    } else if (root.break && root.break.beforeTarget) {
      breakBefore = root.break;
      breakBeforeTarget = breakBefore.beforeTarget;
    } else if (
      root.subform.children.length >= 1 &&
      root.subform.children[0].break &&