make break-inside behavior explicit for table elements such as <tbody>

http://www.w3.org/TR/CSS21/page.html#page-break-props

Currently:

User Agents must apply these properties to block-level elements in the normal flow of the root element. User agents may also apply these properties to other elements, e.g., 'table-row' elements

Although, the specs say that user agents may also apply these properties on elements such as tbody, the word "may" plays its role and the behaviour is not uniform across implementations. This creates some sub-optimal experiences as in order to correctly paginate a table at the tbody element (so headers repeat as expected) the workaround is to add a display:block or display:inline-table style which in turn resets a number of other formatting rules.

See https://stackoverflow.com/questions/27753255/page-break-not-working-with-tbody-issue/27754912#27754912 and https://forum.jsreport.net/topic/1361/page-breaks-in-chrome-pdf-not-appearing-as-expected for examples of why this current behaviour is problematic

Start with the CSS 2.1 page-break properties section linked in the issue, then review the examples showing inconsistent tbody pagination behavior. Define explicit behavior for table elements such as tbody, with wording that resolves the implementation ambiguity without requiring display overrides.