元素有多个类属性？

What happens when an element has multiple class attributes?

当一个元素被声明多次时， ，在行为方面，第一个值将覆盖同一属性的所有后续值。所以在这种情况下，你的元素将只有类一两三。

When an attribute is declared multiple times for a single element (which is invalid HTML, by the way), behavior-wise the first value will override all subsequent values for the same attribute. So in this case, your element will only have the classes one two three.

a href =http://www.w3.org/TR/html5/syntax.html#attribute-name-state> HTML5规范，8.2.4.35属性名称状态，...如果有已经是具有完全相同名称的[element]上的属性，则这是一个解析错误，必须删除新属性...

This behavior is explained in the HTML5 spec, 8.2.4.35 Attribute name state, "... if there is already an attribute on the [element] with the exact same name, then this is a parse error and the new attribute must be removed..."

如果你知道向这个片段（WordPress插件）添加类的正确方法，那么这也是值得赞赏的！

If you know the correct way of adding a class to this snippet (WordPress plugin), then that would also be appreciated!

通常，如果你需要动态添加自定义类到你的WordPress的帖子，你钩上 post_class 过滤器，并操作 $ classes 数组。这里是我的主题大致的样子：

Typically, if you need to add custom classes dynamically to your WordPress posts, you hook onto the post_class filter and manipulate the $classes array as necessary. Here's what it roughly looks like in my themes:

function nv_post_class( $classes ) {
    // Most recent post on the front page
    global $count;
    if ( is_home() && 1 == $count )
        $classes[] = 'latest-post';

    return $classes;
}

add_filter( 'post_class', 'nv_post_class' );

如果只需要添加一个或多个静态类，则直接将它们作为空格分隔的字符串 post_class（）：

If you only need to add one or more static classes, pass them as a space-delimited string directly to post_class():

<div id="post-<?php the_ID(); ?>" <?php post_class( 'myclass1 myclass2' ); ?>>

有关 WordPress Codex 。