* postcss was introduced in #1458 for use within adaptCssForReplay
* #1600 fixes the main case where invalid css could be introduced when if valid css from the output of `sheet.cssRules` was split according to how it was split across text nodes of the <style>
* the guard introduced here is still useful as we likely in future will switch to capturing the raw stylesheet contents (both <style> and <link>), at which point we will be much less confident of getting valid css
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