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 rgbColor = new Uint8ClampedArray(3);
    if (!Array.isArray(color)) {
      this.color = rgbColor;
      return;
    }
    switch (color.length) {
      case 0: 
        this.color = null;
        break;
      case 1: 
        ColorSpace.singletons.gray.getRgbItem(color, 0, rgbColor, 0);
        this.color = rgbColor;
        break;
      case 3: 
        ColorSpace.singletons.rgb.getRgbItem(color, 0, rgbColor, 0);
        this.color = rgbColor;
        break;
      case 4: 
        ColorSpace.singletons.cmyk.getRgbItem(color, 0, rgbColor, 0);
        this.color = rgbColor;