75% was just the first article I pulled up (linked above). Intermittent searches over the last couple years have pulled up similar numbers, usually either 75% or 80%. Realistically, I don't think it's possible to know a hard number. Much like surveys, they only poll a small percentage of a given demographic and base the statistics on that.
WordPress definitely makes up a good portion of that. They claim to power 29% of the internet right on their homepage. To be fair, WordPress is used for a lot of business applications, so there is some development going on there.
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