Empty HTML textarea input doesn't show up in PDF generated by mPDF

I'm generating a PDF from a HTML form using mPDF. The form contains a couple of textarea tags (among other things). If the user doesn't enter anything in one textarea (they don't have to), that textarea doesn't show up in the PDF.

This is what it looks like with text in the textarea:

This is what it looks like without text:

I would like the empty textarea to show up as an empty box in the PDF. It looks fine in the HTML page, but not in the PDF.

I've tried the following:

• Entering a blankspace in the textarea.
• Styling the border with CSS: "border: 1px solid #000".
• Setting the font colour of the textarea to white using CSS ("color: #fff") and adding some text to it. The text is white in the HTML, but in the PDF it shows up as black.

Is there a way to make mPDF render a textarea without text as an empty box, just as it looks in the webrowser (I'm using Firefox btw.)?