Over the past decades, researchers have investigated the effects of multimedia design principles to enhance learning. These evidence-based principles are known to enable students to learn from multimedia resources and support cognitive processing. However, it is unclear if and which of these multimedia design principles are implemented in instructional videos selected by teachers. The purpose of this study is to investigate whether the videos screened by secondary school teachers in their classroom are consistent with the multimedia design principles based on the cognitive theory of multimedia learning. For this study a design review of 78 videos was conducted. The findings indicate variations in the application of multimedia principles. While most of the multimedia design principles were present, some were transgressed or absent. Moreover, the presence of the three different groups of principles, based on their impact on the cognitive load (i.e., reducing extraneous processing, managing essential processing, and fostering generative processing) were rarely present in the same video. Finally, this study also revealed that the teachers were actively implementing the multimedia design principles when they screened the videos in the classroom. Indeed, they frequently applied the pre-training principle (e.g., by introducing new words to the students before the screening), the segmentation principle (e.g., by pausing the video during the screening), and the generative activity principle (e.g., by asking students to complete a task during or right after the screening). Thus, the present study not only provides an insight on the design of authentic instructional videos but also highlights the addition of multimedia principles by teachers during the screening.