Валидатор выдает следующую ошибку - а как это исправить

Warning Line 2, Column 51: character "<" is the first character of a delimiter but occurred as data

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This message may appear in several cases:

You tried to include the "<" character in your page: you should escape it as "<"
You used an unescaped ampersand "&": this may be valid in some contexts, but it is recommended to use "&", which is always safe.
Another possibility is that you forgot to close quotes in a previous tag.