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 color = style.color;
    if (color) {
      if (color === "#000000") {
        
        delete style.color;
      } else if (!color.startsWith("#")) {
        
        
        style.background = color;
        style.backgroundClip = "text";
        style.color = "transparent";
      }
    }
    if (this.baselineShift) {
      style.verticalAlign = measureToString(this.baselineShift);
    }
    
    
    if (this.kerningMode !== "none") {
      style.fontKerning = "normal";
    }
    if (this.letterSpacing) {
      style.letterSpacing = measureToString(this.letterSpacing);
    }
    if (this.lineThrough !== 0) {
      style.textDecoration = "line-through";
      if (this.lineThrough === 2) {
        style.textDecorationStyle = "double";
      }
    }