the users should be informed by a system all the time that people can make better decisions. Match between system and the real world: the systems' language should be similar to users' language. User control and freedom: It happens many times that users choose the wrong system functions by a mistake, therefore, the system needs to contain the "emergency exit" to give an option for users to leave the unwanted state without any problem. Consistency and standards: the users have to be aware that different words, actions and situations can mean the same thing. Error prevention: System should have a careful design that can prevent a problem that can occur in the first place. Recognition rather than recall: the system should have the actions and options visible so the users do not need to memorize everything from previous steps. Instructions about the system usability should be always visible. Flexibility and efficiency of use: the system should have an accelerator to help experienced and inexperienced users to make work faster and easier. Aestetic and minimalist design: the systems needs to contain only relevant and useful information. The information should be clear and short. Help users recognize, diagnose, and recover from errors: the error messages should be presented in a clear language and understandable form (no codes), and suggest the solutions. Help and documentation: the information about help and documentation should be easy to find and should focus on the users' tasks. Web Content Accessibility Guidelines The W3C publishes a set of guidelines on Web accessibility called Web Content Accessibility Guidelines (WGAC). The second revision of WCAG, WCAG 2.0, is composed of twelve guidelines, distilled following the four principles that Web content should adhere to: being Perceivable, Operable, Understandable and Robust. W3C also provides a detailed checklist for this set of guidelines. Universal Usability Challenges To attain universal usability for Web-based services, designers and developers should take technology variety, user diversity and gaps in user knowledge into consideration. Technology variety arises from a broad range of hardware, software, and network access. People are using different hardware and software to access web services. For example, reading a document with smartphone doc reader is different from opening this document with Microsoft Office on a laptop. Also, opening a webpage with 100KB/s bandwidth may lead to failure when it's designed for 2MB/s. Ensuring usability of different platforms can be challenging. User diversity influenced by many