It currently always uses PHP_EOL, which, of course, works, but you get slightly different results in Windows and Unix. User can now set line ending to `\n` or `\r\n` to ensure consistent results regardless of environment. Default remains PHP_EOL. We already do this for CSV.
Fix#4773. Mpdf is applying cell style only to the first line of a multi-line string. Explanation - Html Writer replaces newlines with `<br />` by calling `nl2br`, at least that was the intention. However, `nl2br` doesn't replace - it prepends. This doesn't do any harm in Html, Dompdf, or Tcpdf. However, Mpdf is known to be subject to occasional regexp backtracking errors when parsing large blocks of html. To avoid this, we chunk the html by splitting it at newlines, but this causes the styling to be lost for rich text elements following a newline.
The solution to this problem seems easy enough - replace `nl2br` with a routine that replaces rather than prepends. Unfortunately, Html Reader needs the newlines intact in order to parse things correctly. So a solution where the `nl2br` calls are used for non-Mpdf and using a substitute routine for Mpdf ought to work. However, doing my testing uncovered another bug - rich text with newlines can result in multiple `<br />` tags (see new Html test). The solution to this is a bit kludgey, but it does seem to fix both the Mpdf problem and the newly discovered Html problem. It will also tend to avoid the very minor problem of generating different line endings on Windows systems.
oleibman