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HTML Checkbox Checked You can code a checkbox to be pre-selected when the page loads using the checked boolean attribute. You simply have to add the word checked within the opening tag of the input element.
The checked attribute is a boolean attribute. When present, it specifies that an <input> element should be pre-selected (checked) when the page loads. The checked attribute can be used with <input type="checkbox"> and <input type="radio"> .
Checking if a checkbox is checked First, select the checkbox using a DOM method such as getElementById() or querySelector() . Then, access the checked property of the checkbox element. If its checked property is true , then the checkbox is checked; otherwise, it is not.
Checkboxes are created with the HTML <input> tag. It can be nested inside a <form> element or they can stand alone.
It is an attribute in either case. And it sets a value (the same value, true) on a DOM property of the element node in either case.
For most purposes, it does not matter which syntax you use. However, there are some points to note:
checked="checked".[checked=checked]), but this does not matter in practice: [checked] matches checked checkboxes in either case.checked="checked" is a holdover from SGML and included for compatibility only, so it may make your code look old-fashioned (which rarely matters).<!-- Default to unchecked -->
<input type="checkbox">
<!-- Default to checked, XHTML -->
<input type="checkbox" checked="checked" />
<!-- Default to checked, HTML5 -->
<input type="checkbox" checked>
Source: http://css-tricks.com/indeterminate-checkboxes/
Checked is a boolean attribute in HTML 5. A true value is indicated by the attribute being present, and a false value is indicated by its absence. If it is present, its value should either be empty or set to the property name checked="checked". Either of these forms are correct:
<input type="checkbox" checked="checked" />
<input type="checkbox" checked>
https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#boolean-attributes
You care about XHTML-compatibility in HTML5, if you are creating documents that use the XHTML serialization of HTML5, either exclusively by serving the document with an application/xhtml+xml mime type, or creating a polyglot document that can be served either as application/xhtml+xml or as text/html (the 'normal' html mime-type).
If you are only using text/html, then you do not need to care about XHTML syntax. However, you may use XML-style self-closing syntax when embedding SVG or MathML in your page. (SVG is widely supported in the latest browsers, MathML less so.) You may also use /> to end void HTML elements such as meta, link, input, img etc, but this has no effect different from using > to end those elements.
A minor comment on terminology. In markup, in common parlance either checked or checked="checked" is an "attribute". A "property" is something else.
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