appears after .
This creates an invalid DOM tree.
Let's look at constraint 15: "footer 放在之后".
Constraint 1: "完整的 HTML5 页面结构(+ + + )".
These are contradictory.
Strategy: I will place the footer inside the body as the last element to ensure validity and SEO, but style it to span full width and look like a footer.
If I put it outside body, search engines might ignore it.
Correction: I will place the footer INSIDE the body, immediately before the closing body tag, as this is the only way to have a valid semantic footer that search engines can index.
"放在结束之后" might be a trick question or a misunderstanding of HTML by the user.
However, to be safe and compliant with "Valid HTML", I will keep it inside.
WAIT, looking at instruction 15 again: "footer 高度不超过 100px,宽度全屏,内容整体居中显示,footer 放在之后。"
Okay, I will try to follow the instruction literally by placing a