This sample page demonstrates the idea of Advanced Content Filter (ACF), a sophisticated tool that takes control over what kind of data is accepted by the editor and what kind of output is produced.

When and what is being filtered?

ACF controls every single source of data that comes to the editor. It process both HTML that is inserted manually (i.e. pasted by the user) and programmatically like:

editor.setData( '<p>Hello world!</p>' );

ACF discards invalid, useless HTML tags and attributes so the editor remains "clean" during runtime. ACF behaviour can be configured and adjusted for a particular case to prevent the output HTML (i.e. in CMS systems) from being polluted. This kind of filtering is a first, client-side line of defense against "tag soups", the tool that precisely restricts which tags, attributes and styles are allowed (desired). When properly configured, ACF is an easy and fast way to produce a high-quality, intentionally filtered HTML.

How to configure or disable ACF?

Advanced Content Filter is enabled by default, working in "automatic mode", yet it provides a set of easy rules that allow adjusting filtering rules and disabling the entire feature when necessary. The config property responsible for this feature is config.allowedContent.

By "automatic mode" is meant that loaded plugins decide which kind of content is enabled and which is not. For example, if the link plugin is loaded it implies that <a> tag is automatically allowed. Each plugin is given a set of predefined ACF rules that control the editor until config.allowedContent is defined manually.

Let's assume our intention is to restrict the editor to accept (produce) paragraphs only: no attributes, no styles, no other tags. With ACF this is very simple. Basically set config.allowedContent to 'p':

var editor = CKEDITOR.replace( textarea_id, {
	allowedContent: 'p'
} );

Now try to play with allowed content:

// Trying to insert disallowed tag and attribute.
editor.setData( '<p style="color: red">Hello <em>world</em>!</p>' );
alert( editor.getData() );

// Filtered data is returned.
"<p>Hello world!</p>"

What happened? Since config.allowedContent: 'p' is set the editor assumes that only plain <p> are accepted. Nothing more. This is why style attribute and <em> tag are gone. The same filtering would happen if we pasted disallowed HTML into this editor.

This is just a small sample of what ACF can do. To know more, please refer to the sample section below and the official Advanced Content Filter guide.

You may, of course, want CKEditor to avoid filtering of any kind. To get rid of ACF, basically set config.allowedContent to true like this:

CKEDITOR.replace( textarea_id, {
	allowedContent: true
} );

Beyond data flow: Features activation

ACF is far more than I/O control: the entire UI of the editor is adjusted to what filters restrict. For example: if <a> tag is disallowed by ACF, then accordingly link command, toolbar button and link dialog are also disabled. Editor is smart: it knows which features must be removed from the interface to match filtering rules.

CKEditor can be far more specific. If <a> tag is allowed by filtering rules to be used but it is restricted to have only one attribute (href) config.allowedContent = 'a[!href]', then "Target" tab of the link dialog is automatically disabled as target attribute isn't included in ACF rules for <a>. This behaviour applies to dialog fields, context menus and toolbar buttons.

Sample configurations

There are several editor instances below that present different ACF setups. All of them, except the last inline instance, share the same HTML content to visualize how different filtering rules affect the same input data.

CKEDITOR.replace( 'editor2', {
	allowedContent:
		'h1 h2 h3 p blockquote strong em;' +
		'a[!href];' +
		'img(left,right)[!src,alt,width,height];' +
		'table tr th td caption;' +
		'span{!font-family};' +'
		'span{!color};' +
		'span(!marker);' +
		'del ins'
} );

CKEDITOR.replace( 'editor3', {
	allowedContent: {
		'b i ul ol big small': true,
		'h1 h2 h3 p blockquote li': {
			styles: 'text-align'
		},
		a: { attributes: '!href,target' },
		img: {
			attributes: '!src,alt',
			styles: 'width,height',
			classes: 'left,right'
		}
	}
} );

CKEDITOR.replace( 'editor4', {
	removePlugins: 'bidi,font,forms,flash,horizontalrule,iframe,justify,table,tabletools,smiley',
	removeButtons: 'Anchor,Underline,Strike,Subscript,Superscript,Image',
	format_tags: 'p;h1;h2;h3;pre;address'
} );