In the context of the title, I understood it as "operating systems". This may be due to a few things, but from grammar alone, "are" typically requires a plural

In the context of the title, I understood it as "operating systems". This may be due to a few things, but from grammar alone, "are" typically requires a plural noun, an "Open Sources" makes no sense, so if I was confused by "OS", the "Are" clued it into "Operating Systems" since the plural there makes a lot more sense. "Why Browsers Are Essential to the Internet and How Operating Systems Are Holding Them Back" "Why Browsers Are Essential to the Internet and How Open Sources Are Holding Them Back"