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):

* distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import { XFAFactory } from "../../src/core/xfa/factory.js";
describe("XFAFactory", function () {
  describe("toHTML", function () {
    it("should convert some basic properties to CSS", function () {
      const xml = `
<?xml version="1.0"?>
<xdp:xdp xmlns:xdp="http://ns.adobe.com/xdp/">
  <template xmlns="http://www.xfa.org/schema/xfa-template/3.3">
    <subform name="root" mergeMode="matchTemplate">
      <pageSet>
        <pageArea>
          <contentArea x="123pt" w="456pt" h="789pt"/>
          <medium stock="default" short="456pt" long="789pt"/>
          <draw y="1pt" w="11pt" h="22pt" rotate="90" x="2pt">
            <font size="7pt" typeface="FooBar" baselineShift="2pt">
              <fill>
                <color value="12,23,34"/>
                <solid/>
              </fill>
            </font>
            <value/>
            <margin topInset="1pt" bottomInset="2pt" leftInset="3pt" rightInset="4pt"/>
            <para spaceAbove="1pt" spaceBelow="2pt" textIndent="3pt" marginLeft="4pt" marginRight="5pt"/>
          </draw>
        </pageArea>