Что делает Прагму HTTP-заголовка: средняя Общественность?

According to the standard, Pragma is implementation dependent (section 14.32), except for no-cache because of its wide use. Cache-Control (section 14.9) is the proper way to control caching. This is what the standard says for a Cache-Control: public:

Indicates that the response MAY be cached by any cache, even if it would normally be non-cacheable or cacheable only within a non- shared cache.