In file https://github.com/mozilla/pdf.js/files/5249872/product1.pdf, the fifth text widget has a default appearance using font F1 and this one is neither in DR nor in AcroForm but in AP::N::Resources.
const glyphs = font.charsToGlyphs(text);
    const scale = fontSize / 1000;
    let width = 0;
    for (const glyph of glyphs) {
      width += glyph.width * scale;
    }
    let shift;
    if (alignment === 1) {
      
      shift = (totalWidth - width) / 2;
    } else if (alignment === 2) {
      
      shift = totalWidth - width - hPadding;
    } else {
      shift = hPadding;
    }
    shift = shift.toFixed(2);
    vPadding = vPadding.toFixed(2);
    return `${shift} ${vPadding} Td (${escapeString(text)}) Tj`;
  }
  /**
   * @private
   */
  _getSaveFieldResources(xref) {