CSS glossary entries for ‘R’

Relationship selectors: Are CSS selectors which target an element based on its relationship to one or more other elements, i.e. the targeted element has a defined relationship to the other elements specified within the structure or semantics of an HTML document. For example, the selected element is a ‘child’, ‘grand Child’, ‘sibling’ of the previous element specified (or linked to it via the for attribute). Relationship selectors always consist of 2 or more individual selectors and are, therefore, also known as ‘combinators’. The simplest relationship selector is the ‘Descendant Selector’, which selects an element if it is contained (at any level) within another specified element. Alternative relationship selectors include: the ‘Child Selector’, the ‘Directly Adjacent Selector’, the ‘Indirectly Adjacent Selector’, the ‘Reference Combinator Selector’ and the ‘Subject Selector’
Rendering engine: A rendering engine is the component of a web browser which draws the content of a web page on a screen (or analogous device) following the ‘rules’ of a stylesheet and hints provided by the HTML markup.
Required selector: The required selector (E:required) is a CSS3 ‘pseudo-class selector’ which matches form elements with a required attribute in their opening tag. It is typically used to add styling clues and highlights to draw the user's attention to the fact that they must complete the given form components in order to successfully submit the form.
right: The right property is a CSS2 property which sets the offset distance between the right edge of a ‘positioned element’ and the right edge of its ‘containing element’.
Root selector: The root selector (E:root) is a CSSi3 pseudo-class which matches the root element of a web page ‘DOM’, i.e. the <html> element. The only difference between using the root selector and the ‘element type selector’ for html is that it has higher ‘specificity’, i.e. a CSS ‘rule’ with a root selector can override rules with html element type selectors, even if they occur after it in the ‘cascade’.
Rules: A rule is the fundamental unit of CSS syntax. Every CSS styling instruction is expressed as, or within, a rule. Every rule has 2 components: a ‘selector’ and a ‘declaration’.