Climate change is having a negative impact on the environment, with the ranges of plants and animals shifting, sea levels and temperatures rising, and glaciers melting. Although multiple actions are being considered, the negative impact of carbon emissions on the environment and society are still there. Especially in the Information and Communication Technology (ICT) domain, activities performed on any platform, application, or service requires the use of internet, servers, data centers, or storage warehouses, leading to the production of additional carbon emissions. It is true that the virtual world has a significant impact on the actual world, but how can an application User Experience (UX) and User Interface (UI) designer contribute to this total impact? In this context, the purpose of this manuscript is to provide the principles towards a sustainable environment, analyzing the research efforts towards designing web applications' UX/UI that are environmentally sustainable, and do not produce significant amounts of CO2 emissions. A list of best practices is provided towards these goals, which are being evaluated on top of a web application whose UX/UI has been designed in two (2) versions respecting - or not - these practices. The energy consumption and carbon footprint for both cases is measured, concluding that the provided list of practices is efficient and can be considered as a promising starting point towards a more sustainable future.