When delivering content within a website it is best practice that the data itself and the presentation layout are distinguished or separated. For websites, this is done using the Cascading Style Sheets (CSS) standard. CSS standard defines a language to apply look-and-feel (e.g., color, font-size, position of elements, etc.) to any given markup establishing an appearance on a user-interface. A related CSS style definition consists of a selector, declaring which of the markup elements the style applies to, and a declaration block of property-value pairs. Selectors may be based on a particular type of markup element (e.g., h1, i.e., all first order headings), a CSS class (e.g., .customStyle, i.e., all elements tagged with the CSS class customStyle), or a combination of both. The property-value pairs typically contain actual layout definitions of the style, like color:black, i.e., a black color is used for displaying.
CSS-styles are typically defined on three different levels:                1. Inline style defined directly for a particular markup element, in which case the selector is omitted (e.g., <p style=“color:blue;margin-left:10 px”>);        2. Embedded style definitions in markup: a text block of CSS definitions enclosed by the <style> tag; and        3. External style sheet files: a separate file containing CSS definitions referenced from the markup, e.g. <link rel=“stylesheet” href=“style.css” type=“text/css”>        
The CSS specification itself defines the following evaluation order for style definitions:                Inline style definitions over-rule embedded or external style sheets, meaning that inline styles are always considered as definitive, whereas embedded and external CSS definitions are applied with lower priority and treated equally.        More specific selectors over-rule less specific selectors—e.g., if there are definitions for “h1 .customStyle” and “h1”, any h1 element tagged with the customStyle class is styled according to the definitions of “h1 .customStyle”.        
The mechanisms of CSS work well in environments where the website operator or designer has control over the styles that are being delivered. For solutions that aggregate markup from multiple sources, e.g., like portal environments, several problems discussed in context of FIG. 2 may occur.