In general, some research points out the lack of tools and systems in virtual learning environments for the application of accessibility metadata. In this topic, [
36] established the lack of reviews related to the issue of accessibility by the different educational platforms. However, it is worth noting the existence of literature on the analysis of limited scope on some courses. Batanero et al. [
33] recommended the application of standards based on the general analysis of web accessibility and the level of compliance with the WCAG standard. The understanding of the different standards in the area of accessibility and adaptability leads to future studies focused on developing evaluation mechanisms, automatic tools, and methodological proposals in order to contribute to the development of accessibility in e-learning. It is important to measure the impact on the user experience [
55] considering the use of assistive technology [
59]. Another great challenge is to provide students with advanced and personalized services to efficiently manage and disseminate educational material [
65]. The difficulty of effectively interpreting the content of educational resources that facilitates personalization constitutes a research challenge. Lin et al. [
62]. It is worth building systems with intuitive adaptations that facilitate the delivery and selection of educational resources for students with disabilities [
30,
46]. In addition, the generation of instructional design methodologies is important because it supports the management and dissemination of educational material according to specific needs [
58]. The development of tools that strengthen the applicability of accessibility metadata is also required. There exist frameworks based on semantic rules that facilitate the self-personalization of assistive technologies [
48]. However, other mechanisms are necessary for the recommendation of learning objects, e.g., self-organization strategies [
56] and self-control [
61]. Recommender systems are also needed to refine the modeling of user profiles in order to establish accessibility requirements for courses and develop a holistic approach [
47], or based on the learning profiles of other students with similar needs or preferences [
66].
The establishment of good accessibility practices in a teaching-learning process is time consuming, since it is necessary to establish pre- and post-comparisons. Rodriguez-Ascaso et al. [
50] present projects of 48 months to address the needs of Accessible Lifelong Learning, as the EU4ALL project that finally lasted 54 months. Additionally, the complexity to design adapted applications is determined due to the wide range of characteristics in the functional diversity of students and the lack of reliable specific data [
31]. Considering that metadata come from multiple and heterogeneous sources, metadata are compiled with different approaches and used in different contexts [
60]. However, the ability to discover resources that do not adjust to a common standard is difficult [
37,
67]. Pal et al. [
63] determines the need to establish a generic model that leads to the use of a universal ontology for all educational domains, which could categorize all the metadata of different topics and subjects. However, there is a lack of a methodological framework for the implementation of an accessible virtual educational project. Therefore, it is necessary to incorporate metrics based on accessibility indicators that facilitate the evaluation of the results obtained in the different phases [
39]. The involvement of teachers in the use and creation of educational resources, their dynamic characteristics and organizational structures are also necessary [
68]. The sustainability and scalability of an educational resource depend on the socialization of the optimal use of repositories and reuse of resources according to the needs of the teaching-learning process. The quality of a resource, including its metadata and accessibility information, represents a broad topic that needs to be integrated [
32]. Although some parameters to measure quality are in competition with each other, both peer review tools and user evaluation tools are methods to evaluate the quality of resources. Thus, [
69] suggests three dimensions of analysis: priority, possibility of achieving (responsibility) and sustainability. Although the use of HTML5 is expected to increase in some areas including education [
53], the implementation of accessibility metadata in educational resources is still incipient. According to [
64], a mapping with eight dimensions of research on the issue of accessibility and MOOCs is proposed, establishing important guidelines in future research. In addition, more research is needed on automatic procedures and policy measures to support and monitor learning activities that involve a massive number of students [
12], also considering other areas of knowledge such as exact sciences and required adaptations [
52]. Future search processes could be facilitated through metadata editors capable of interpreting and reading files generated by various tools [
34]. For example, [
57] presents a conceptual framework for the automated generation of metadata that highlights the importance of adaptive e-learning process based on the learning activity profile. It is also necessary to have automatic support tools that facilitate the use of accessibility metadata [
49] in order to detect drawbacks and successes and propose alternatives in order to increase accessible educational resources in various repositories. In response to RQ4, it can be said that the field of accessibility is extensive, so the techniques and associated standards need to be homogenized and socialized to boost their use among e-learning resource developers. The use of metadata needs to be extended to accessible educational resources, in such a way that the learning curve can be reduced. Additionally, a common language in the development of accessible digital resources with easy implementation and search on the web should be developed.