The :invalid selector selects form elements with a value that does not validate according to the element’s settings.

The :invalid selector only works for form elements with limitations, such as input elements with min/max attributes, email fields, or number fields without a valid numeric value.

/* selects all form input fields that are invalid */
input:invalid {
    box-shadow: 0 0 1px 1px red;
}

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To select multiple HTML elements with varying ids and classes, separate them with commas.

#intro, #footer, .my_paragraphs, h3 {
     background: white;
}

To be more specific when selecting an HTML element, you can refer to the element and an ancestor element (.grandfather .child or #father .child or .grandfather h1 or any number of combinations). Note that there is no comma between the classes (.grandfather .child) which says you are targeting the class .child and it must have an ancestor with the class .grandfather. You can include as many ancestors as necessary to be as specific as necessary.

.grandfather #son .child p {
     background: white;
}

To select an HTML element that may have multiple classes attached, you can be more specific by including more than one class (or even adding an #id or tag name) to point to that one element: .my_paragraph.secondary_paragraph { }. Note that there are no spaces between the classes because they belong to the same element. This would ONLY select an element that has both classes: .my_paragraph and .secondary_paragraph.

h1.intro, p#footer, h1#header.primary, #sidebar#topper.image {
     background: white;
}

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