The Quakers, as is often said, kept wonderfully detailed records, especially for marriages, births, and burials. The published sources for these  early vital records are readily available in libraries, but lack details that you might want to see. For example, they do not include the full marriage certificates, with the list of witnesses (often providing names of close family members). They might change the list of births or deaths from chronological to alphabetical order, causing you to miss details such as clusters of deaths in the summer months.  If you use them as a starting point to find your ancestors in the records, consider browsing the images in Ancestry to find more context. See the Ancestry files for Bucks and Philadelphia Counties, and for Chester County (including meetings that are now in Delaware County). Note that there are no published sources for the vital records of Abington Monthly Meeting.

If you are wondering how a marriage could be recorded as early as 1670, before there were any Quakers in Pennsylvania, some Friends brought their certificates with them from England and had them recorded by their new meeting. Like later marriage certificates, these early ones provide clues about the family and friends of the newly-married couple.

Published sources for early vital records