The <input type=”checkbox”> defines a checkbox.

The checkbox is shown as a square box that is ticked (checked) when activated.
Checkboxes are used to let a user select one or more options of a limited number of choices.

<input type="checkbox">

The HTML checkbox input element allows you to select a single value for submission in a form; for example, if you are creating a form and want to know if the applicant is fluent in English or not you can have a checkbox so the user can check it if not leaves it unchecked.

<form action="">
<label for="name">Name:</label>
<input type="text" name="name"><br>
<label for="language">Do you speak English fluently?</label>
<input type="checkbox" id="fluency" checked />
</form>

What is the value in a checkbox?

A DOMString representing the value of the checkbox. This is never seen on the client-side, but on the server, this is the value given to the data submitted with the checkbox name.
What are the additional checkboxes attributes?

In addition to the common attributes shared by all <input> elements, “checkbox” inputs support the following attributes: checked and value.

How to render checkboxes checked by default?

When rendering a page with a checkbox, you want to be selected or checked by default you need to include the ‘checked’ attribute. There is no required value for the checked attribute.

However, per the checkbox specification, only an empty value or ‘checked’ is valid.

When checked is added to the checkbox element, the browser will render it as selected.
What is the indeterminate state in HTML5 checkboxes?

There is a third state a checkbox can be in beside checked and unchecked, indeterminate. This can only be set via JavaScript and causes the checkbox to render with a horizontal line in the box instead of just a box or a box with a check.

You may be wondering when you might want to use this state?

The indeterminate state is commonly used when there are multiple child or sub-options available. You can set the state to indeterminate when a minimal number of child options are not selected.

How to validate a checkbox?

Checkboxes do support validation (offered to all <input>s). However, most of the Validity States will always be false. If the checkbox has the required attribute but is not checked, then ValidityState.valueMissing will be true.