Extensible Stylesheet Language Formatting Objects (XSL-FO)

XML for print

Not quite the browser, but... XSL-FO is aimed at print output, not Web viewing, but it offers a convenient mechanism for converting on-screen documents to printouts - if the source is in XML and a stylesheet is provided.
A different conception of pages Web 'pages' can have arbitrary length and width. Paper documents have a variety of fixed sizes and a set of tools for use in coping with their size and sequence issues. XSL-FO is about print pages.
PDF, or not Developers can use XSL-FO to create PDFs. If a document is produced in an XML format, it's possible to use one XSLT transformation to produce HTML and another to produce XSL-FO, distributed to the reader as PDF.