Z-index in html2pdf

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I am trying to build an invoice template with http://www.simpleinvoices.org which has implemented html2pdf. I have read the documentation and at http://www.tufat.com/docs/html2ps/compatibility.css.2.1.html I have found that the z-index compatibility is "partial, Supported for absolute-positioned boxes only; only numeric values are supported".My layout needs a watermark image (it