Remove 3D inset border in Webkit for checkboxes when using wkhtmltopdf

Note: this is not a duplicate of other questions about removing checkbox border in browsers. This specifically applies to Webkit as used in wkhtmltopdf, and not as used in Google Chrome.

I am using wkhtmltopdf, an HTML-to-PDF converter that uses the webkit engine.

In the resulting PDF, checkboxes look like this, with an inset border:

I want them to have just a normal 1px solid border.

Here's what I've tried:

input[type=checkbox] {
    border: 1px solid black;
    box-shadow: none;
    /* -webkit-appearance: none; */ /* doesn't work - removes checkbox entirely */
}

What's baffling is that Google Chrome also uses Webkit, but the checkbox looks good there.

Is there any other way to change how the checkbox is displayed by wkhtmltopdf?