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):

["font-family", value => getFonts(value)],
]);
const spacesRegExp = /\s+/g;
const crlfRegExp = /[\r\n]+/g;
function mapStyle(styleStr) {
  const style = Object.create(null);
  if (!styleStr) {
    return style;
  }
  for (const [key, value] of styleStr.split(";").map(s => s.split(":", 2))) {
    const mapping = StyleMapping.get(key);
    if (mapping === "") {
      continue;
    }
    let newValue = value;
    if (mapping) {
      if (typeof mapping === "string") {
        newValue = mapping;
      } else {
        newValue = mapping(value);
      }
    }
    if (key.endsWith("scale")) {
      if (style.transform) {
        style.transform = `${style[key]} ${newValue}`;
      } else {
        style.transform = newValue;
      }
    } else {
      style[key.replaceAll(/-([a-zA-Z])/g, (_, x) => x.toUpperCase())] =