page-break-before:always is supported by the Kindle platform. However there can be problems if you use calibre.
Enhanced Typesetting (KFX format) handles it by splitting the content into a new section (the KFX equivalent of a new HTML file) at that point.
Conversion using Amazon's kindlegen adds <mbp:pagebreak/> in MOBI7 and <div data-AmznPageBreak="always"/> in KF8 to make the page break actually function.
Conversion using calibre handles page breaks slightly differently. Calibre will change page-break-before:always into a split of the HTML file in KF8.
There is a bug in calibre's editor. It does not handle data-AmznPageBreak="always" properly resulting in broken page breaks if you attempt to edit a KF8 format book that was produced using kindlegen. Also you can't just add page-break-before:always into a KF8 book using calibre's editor since it does not convert it into data-AmznPageBreak="always" when you save the file.