doctype

Does my HTML5 doctype have to be on the VERY first line?

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I’ve always wondered whether whitespace before a DOCTYPE matters, but I can’t seem to find any definitive information on the web. People say not to do it, but I can’t see what effect it’s likely to have, or even if the spec says it must be like that. I can’t figure out if it triggers compatibility mode in IE or anything annoying like that.