可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试):

问题:

Take for instance this XHTML snippet:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <title>A webpage</title>
</head>
<body>
  <p>
    <form action="something.php" method="get">
      <input type="submit" value="Hello"/>
    </form>
  </p>
</body>
</html>

The tree should be valid, however this won't parse correctly in a browser.

回答1:

Look at the error messages that you get when you try that with http://validator.w3.org

Apart from a warning that you haven't specified a character encoding (and that it's therefore assuming UTF-8), the main error is that a <p> isn't allowed to contain non-inline content. You can either remove the <p> and </p> completely, or, move them inside the <form>.

As for 'why', it's because that's how it's defined in the schema which defines what is and what is not valid XHTML. If you look at this section of the XHTML definition you'll see that <p> is only allowed to contain text or 'inline' (not 'block') tags. However a <form> counts as 'block' content not as 'inline' content.

In other words, a form can contain paragraphs, but a paragraph cannot contain forms.

回答2:

According to this, because:

Line 8, Column 44: document type does not allow element "form" here; missing one of "object", "ins", "del", "map" start-tag ✉ 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").

Line 9, Column 40: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag input type="submit" value="Hello" 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").

回答3:

You can't put form inside paragraph. Write like this:

<body>
  <form action="something.php" method="get">
  <p>
    <input type="submit" value="Hello"/>
  </p>
  </form>
</body>

回答4:

Try using the W3C Markup Validator, which will tell your what is invalid with most (X)HTML and CSS documents.

回答5:

I ran a test with your HTML code through validator.w3.org and the result is that you need to swap the P and FORM tags for it to pass as valid W3C HTML.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <title>A webpage</title>
</head>
<body>
  <form action="something.php" method="get">
    <p>
      <input type="submit" value="Hello"/>
    </p>
  </form>
</body>
</html>