What is the meaning of WordPress's recommended css classes and where are they applied?

Question

I am creating a theme and when I was checking my theme in the theme check plugin, I saw a lot of errors about css classes. They say some of the classes are required in the theme.

These css classes are as follows (which I have found in WordPress Codex).

/* =WordPress Core
-------------------------------------------------------------- */
.alignnone {
    margin: 5px 20px 20px 0;
}

.aligncenter,
div.aligncenter {
    display: block;
    margin: 5px auto 5px auto;
}

.alignright {
    float:right;
    margin: 5px 0 20px 20px;
}

.alignleft {
    float: left;
    margin: 5px 20px 20px 0;
}

a img.alignright {
    float: right;
    margin: 5px 0 20px 20px;
}

a img.alignnone {
    margin: 5px 20px 20px 0;
}

a img.alignleft {
    float: left;
    margin: 5px 20px 20px 0;
}

a img.aligncenter {
    display: block;
    margin-left: auto;
    margin-right: auto;
}

.wp-caption {
    background: #fff;
    border: 1px solid #f0f0f0;
    max-width: 96%; /* Image does not overflow the content area */
    padding: 5px 3px 10px;
    text-align: center;
}

.wp-caption.alignnone {
    margin: 5px 20px 20px 0;
}

.wp-caption.alignleft {
    margin: 5px 20px 20px 0;
}

.wp-caption.alignright {
    margin: 5px 0 20px 20px;
}

.wp-caption img {
    border: 0 none;
    height: auto;
    margin: 0;
    max-width: 98.5%;
    padding: 0;
    width: auto;
}

.wp-caption p.wp-caption-text {
    font-size: 11px;
    line-height: 17px;
    margin: 0;
    padding: 0 4px 5px;
}

/* Text meant only for screen readers. */
.screen-reader-text {
    clip: rect(1px, 1px, 1px, 1px);
    position: absolute !important;
        white-space: nowrap;
    height: 1px;
    width: 1px;
    overflow: hidden;
}

.screen-reader-text:focus {
    background-color: #f1f1f1;
    border-radius: 3px;
    box-shadow: 0 0 2px 2px rgba(0, 0, 0, 0.6);
    clip: auto !important;
    color: #21759b;
    display: block;
    font-size: 14px;
    font-size: 0.875rem;
    font-weight: bold;
    height: auto;
    left: 5px;
    line-height: normal;
    padding: 15px 23px 14px;
    text-decoration: none;
    top: 5px;
    width: auto;
    z-index: 100000; /* Above WP toolbar. */
}

I do not understand where those classes will be applied in the theme by WordPress when I didn't put any class like that in my theme.

Answer 1

These classes are generated by WordPress. For instance, if a user inserts an image in his post and says it must be aligned right, WP will generate code like this in the post:

<img class="alignright" src="...">

Now, if your theme doesn't define the alignright class, the image won't be aligned right and the user won't get what he expects.

You can simply copy these styles from the core and you'll be fine. But you may also want to adapt things, such as the color of the border around an image caption.

The last two classes are there to help visually impaired people. So, if you're using a regular screen, nothing will happen. But it's helping others.

You may also want to read this answer by Chip Bennett on other classes that are generated by WordPress to give you more styling possibilities.