In the older version of HTML, we need to write a long like to declare DOCTYPE because HTML is SGML(Standard Generalized Markup Language) based so its need a DTD (document type definition).

There is an error while package project to HTML5： UATHelper: Packaging (HTML5): XXX: -o D:\resource\UE\src\UnrealEngine-4.27-html5_dev\Engine\Binaries\HTML5\UE4Game ...