什么是HTML，看完这篇文章就懂了

气象公报 ＞＞ 交通气象预报

　　Despite the name , pragma directives are 　　almost entirely unrelated to HTTP headers. Implementers and web developers are best off thinking 　　of them as entirely separate, and the name as being a historical accident. 　　In more detail, although the 　　keyword has the same processing model as the corresponding `` header, every 　　other standardized pragma directive has at least slightly different behavior than the 　　similarly-named header. (And usually dramatically-different behavior.) 　　Implementers or specification writers contemplating adding new document-level pragmas or HTTP 　　header-controlled switches should be cautious about this mismatch, and avoid perpetuating the 　　existing confusion by adding the same or similar behavior to both an HTTP header and an pragma. Instead, consider providing only an HTTP 　　header, or if an in-document pragma is needed, consider adding a new attribute to 　　similar to the model used by the attribute. (Note that avoiding in-document pragmas is 　　often the better choice, since the DOM is mutable. Thus, even in the simple case where the 　　developer does not add, remove, or mutate elements, the policy will go from 　　un-applied to applied during parsing, which can have complex implications.)

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