https://ckeditor.com/docs/ckeditor4/latest/guide/dev_allowed_content_rules.html
extraAllowedContent: '*[*]{*}(*)',
elements [attributes]{styles}(classes)// A rule accepting <p> and <h1> elements, but without any property.
p h1
// A rule accepting <p> and <h1> elements with optional "left" and "right" classes.
// Note: Both elements may contain these classes, not only <h1>.
p h1(left,right)
// A rule accepting <p> and <h1> elements with all their attributes.
p h1[*]
// A rule accepting <p> and <h1> elements with all their attributes
// starting from 'data-'.
p h1[data-*]
// A rule accepting <a> only if it contains the "href" attribute.
a[!href]
// A rule accepting <img> with a required "src" attribute and an optional
// "alt" attribute plus optional "width" and "height" styles.
img[alt,!src]{width,height}
// The same as above, because the order of properties and their lists is irrelevant and white-spaces are ignored.
img { height, width } [ !src, alt ]// Rules allowing:
// * <p> and <h1> elements with an optional "text-align" style,
// * <a> with a required "href" attribute,
// * <strong> and <em> elements,
// * <p> with an optional "tip" class (so <p> element may contain
// a "text-align" style and a "tip" class at the same time).
p h1{text-align}; a[!href]; strong em; p(tip)
// Rules allowing:
// * <p> and <h1> elements with an optional "id" attribute,
// * <a> with a required "href" attribute and an optional "id" attribute.
p h1; a[!href]; *[id]