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Text runs must be wrapped according to the CSS line-wrapping rules, with the following additional constraints: 
* Regardless of the value of the 'white-space' property, lines must be wrapped at the edge of their containing blocks, even if doing so requires splitting a word where there is no line breaking opportunity. (Thus, normally text wraps as needed, but if there is a particularly long word, it does not overflow as it normally would in CSS, it is instead forcibly wrapped at the box's edge.)
* Regardless of the value of the 'white-space' property, any line breaks inserted by the user agent for the purposes of line wrapping must be placed so as to minimise Δ across each run of consecutive lines between preserved newlines in the source. Δ for a set of lines is defined as the sum over each line of the absolute of the difference between the line's length and the mean line length of the set.
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We are not happy with this requirement. There are several problems.

The first bullet point is already available in CSS with word-wrap:break-word. We don't want to have two *slightly* different ways to force word wrapping just because the requirement here is differently worded than the CSS feature. Just say that word-wrap:break-word be applied by default (and this property could be white-listed).

We don't want to wrap when white-space is pre or nowrap. We basically don't want to derivate from CSS behavior.

The balanced line wrapping rules is overly precise. Since it is largely undefined in CSS where line break opportunities are, it seems inappropriate to have precise rules for balancing, and UAs should be allowed to approximate the balancing for perf.

So, what we want is:

* Remove the quoted spec text.
* Add word-wrap:break-word; as a default style.
* Relax the balancing algorithm.