Due to the market share of modern browsers (depending on the target market), the compatibility of a website with the viewers is restricted. For instance, a website that is designed for the majority of web surfers will be limited to the use of valid XHTML 1.0 Strict or older, Cascading Style Sheets Level 1, and 1024x768 display resolution.. This is because Internet Explorer is not fully W3C standards compliant with the modularity of XHTML 1.1 and the majority of CSS beyond 1. A target market of more alternative browser (e.g. Firefox and Opera) users allow for more W3C compliance and thus a greater range of options for a web designer.

Usage share, in web browser statistics, is the percentage of visitors to a group of web sites that use a particular browser. For example, when it is said that Internet Explorer has 83% usage share, it means that Internet Explorer is used by 83% of visitors that visit a given set of sites. Typically, the user agent string is used to identify which browser a visitor is using.
The Markup Validation Service by the World Wide Web Consortium, W3C, allows internet users to check HTML documents for conformance to their HTML and XHTML standards

The display resolution of a digital television or computer display typically refers to the number of distinct pixels in each dimension that can be displayed

Another restriction on webpage design is the use of different Image file formats. The majority of users can support GIF, JPEG, and PNG (with restrictions). Again Internet Explorer is the major restriction here, not fully supporting PNG's advanced transparency features, resulting in the GIF format still being the most widely used graphic file format for transparent images.

Image file formats provide a standardized method of organizing and storing image data.
GIF (Graphics Interchange Format) is an 8-bit-per-pixel bitmap image format that was introduced by CompuSserve in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability.

CompuServe, (in full, CompuServe Information Services, or CIS), was the first major commercial online service in the United States.

In computing, JPEG is a commonly used standard method of compression for photographic images. The name JPEG stands for Joint Photographic Experts Group.

PNG (Portable Network Graphics) is a bitmapped image format that employs lossless data compression.

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data
Many website incompatibilities are not noticed by the designer and not reported by the users. The only way to confirm a website works on a particular platform is to test it on that platform.