htmlファイルのバリデートに関してです．
htmlを書くときにW3Cで定められたいろいろなルールがあると思います．
例えば，インライン要素の中に，ブロック要素を入れてはいけないなどです．
ですが，以下の二つを実際に表示してみると，クリック可能な範囲が異なるものの，どちらも表示されます．
（h1要素はブロック要素，a要素はインライン要素です．したがって，h1要素内にa要素を入れるのがW3C的には正しい）
<h1>
<a href="#">リンク</a>
</h1>
<a href="#">
<h1>リンク</h1>
</a>
このようなものでも，
にバリデートをかけたいhtmlファイルをアップデートすると，書式が間違っている箇所を教えてくれます．
やってみると，「あ，この書き方だめだったんだ．知らなった！」のようなこともあるので，ぜひ試してみてください．
ちなみに，上の例(a要素内にh1要素)だと，
Error Line 22, Column 16: document type does not allow element "a" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag
<a href="#">The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
One possible cause for this message is that you have attempted to put a block-level element (such as "
<p>" or "<table>") inside an inline element (such as "<a>", "<span>", or "<font>").
と、
Error Line 23, Column 12: document type does not allow element "h1" here; missing one of "object", "ins", "del", "map", "button" start-tag
<h1>link</h1>The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
One possible cause for this message is that you have attempted to put a block-level element (such as "
<p>" or "<table>") inside an inline element (such as "<a>", "<span>", or "<font>").
のように，インライン要素の中に、ブロック要素をいれるな！！とちゃんと注意してくれます．