这个答案适用于XHTML，而不是HTML5。 b
$ b

form和body元素只接受
块级别的子元素< form action =/>
< input type =submit> < / form> ...将
产生错误：

文档类型不允许元素
输入这里;缺少p，
h1，h2，h3，h4，h5，h6，
div，pre，address ，fieldset，
ins，delstart-tag。
$ b

在（X）HTML的严格变体中，
元素可能只有块元素
作为其子元素，但表单控件
（例如输入元素）是内联
元素。解决方案是选择一个
块元素，并使用适当的
语义可能包含内联
元素;有用的验证器
产生一个列表，可以帮助你减少
的范围。

当涉及到一个表单时，适当的
元素通常是字段设置或
普通div。

Source - Dorward Online

 < form action =form.phpmethod =post> 
< fieldset> 
< input type =text/> 
< / fieldset> 
< / form> 
 

可以解决您的问题。

I got this error when validating my page with w3c's validator.

Source:

<form action="form.php" method="post">
<input type="text"/>
</form>

Can someone show me why I may have gotten this error? Thanks in advance!

This answer applies to XHTML, not HTML5.

The form and body element only accept block level children <form action="/"> <input type="submit"> </form> … will produce the error:

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.

In Strict variants of (X)HTML, a form element may have only block elements as its children, but form controls (such as input elements) are inline elements. The solution is to pick a block element with appropriate semantics that may contain inline elements; helpfully the validator produces a list that can help you narrow it down.

When it comes to a form, appropriate elements are usually fieldset or a plain div.

Source - Dorward Online

So doing something like

<form action="form.php" method="post">
    <fieldset>
        <input type="text"/>
    </fieldset>
</form>

will solve your problem.