Why can't ajax load html be displayed locally?

when index.html loads header.html through ajax, why is it accessed locally and cannot be displayed by header, but only when it is accessed by IP? what is the principle?

Mar.14,2021

access through local files. Browsers disable reading of local files by default

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b382c2-2ae37.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b382c2-2ae37.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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