CSS expressions are a method to dynamically set CSS properties, like background color, page width, font properties, page border, and about 100+ other properties. They were initially introduced with Internet Explorer 5, which was released in 1999. CSS properties are fantastic for web page designers, allowing them to customize and display web pages in a manner that supports the theme and feel of their specific business. It provides a way for web designers to set their pages apart from the competition. For example, with a CSS expression, you could create an expression that would change the background image of a web page to a distinct color every hour.
However, while these CSS expressions were great in help to provide a way to set your website’s visualizations, they can hinder web page load speed. Additionally, with CSS, what works in one browser, may not work in another, so not only would do you have to worry about slow load times, you also must consider that some CSS properties may not be supported at all. These factors led to CSS expressions being removed from all modern browsers except Internet Explorer IE6 and IE7.