## 2 Background
### Accessibility in Serious Games  
As we have mentioned, accessibility [12] is the inclusive practice of ensuring that serious games and technologies are designed and developed so that people with disabilities can use them. In the context of serious games, accessibility includes aids such as high-contrast or large font size, screen magnifiers, screen readers and reviewers, voice recognition software, keyboards, and adapted devices according to user needs.  
The study [26] argues that application designers face a significant challenge in applying accessibility guidelines because they are complex and ambiguous. For this reason, the authors propose following the WCAG with a user-centered approach. They offer designers an innovative and engaging solution for broader adoption of accessibility guidelines that can create accessible games.  
The study [27] indicates that games offer a traditional channel where all people can be involved; however, this only happens when creating inclusive design environments to achieve this challenge. Therefore, they suggest developing an accessibility vocabulary and language for games. The authors recommend including the terminology used, showing the controls that enable the play's tasks and skills.  
The investigation [28] indicates that play accessibility is about removing barriers for users with disabilities. Besides which, the authors argue that the challenge is that web-based games can better access the WCAG 2.1 and achieve an adequate accessibility level in the construct of more accessible games.  
Previous studies by the study [29] indicate that it is essential to evaluate the users' learning styles, for which they applied the Honey-Alonso test [30] for the development of serious games. The Honey-Alonso test includes active, reflective, theoretical, and practical techniques. The authors proposed constructing an educational video game with a Honey-Alonso test script and applied the WCAG.  
The research [31] argues that making serious games universal and accessible are a challenge for interface creators and game designers. The authors indicate that more effort is needed and propose research with recommendations for accessibility in serious games. Consequently, the authors recommend ten strategies for designing games for visually impaired users. The results indicated that only three out of ten games were accessible.  
Concerning accessibility, in June 2018, the World Wide Web Consortium (W3C) [32] established the WCAG 2.1 recommendations for making web content more accessible; furthermore, this proposal can be applied to achieve accessibility in serious games. WCAG 2.1 covers a wide range of recommendations to make web content more accessible. Applying [12] these guidelines will make serious game content more accessible to people with disabilities, including adaptations for blindness and low vision, deafness and hearing loss, limited movement, speech disabilities, photosensitivity, and combinations, and some adaptations for learning disabilities and cognitive limitations. WCAG 2.1 comprises four principles, 13 guidelines, 78 compliance criteria, combined with an undefined number of sufficient techniques and advisory techniques. The four principles are described below:  
1. [label=(0)]
2. Perceivable, users must distinguish content in audio, video, and text.
3. Operable; users must be able to use and interact with the interface modules.
4. Understandable, users must be able to recognize self-controls of the interface.
5. Robust, users must be able to access comfortably and understand consistently in any case of the present and potential technologies.  
Therefore, the compliance criteria include three levels. Level A offers a minimum of accessibility; not achieving implies that users cannot access the content. Level AA allows an intermediate level of accessibility; it satisfies the levels A and AA; it indicates that the users cannot access all the content. Furthermore, Level AAA is the highest level of accessibility. Achieving this level means that users can easily access the content.