If you are asked, in an exercise to refer to a file with file name dodona.png, for example Dodona.png and dodona.jpg will not work. If you are asked to provide a header with the text UGent, Ugent is not correct. So pay attention to capital letters, spaces and spelling mistakes. Dodona usually compares whether your text exactly matches with the model solution.
Use this sources to find the correct HTML-tags:
If the problem contains hints, they will point to that website as well.
There is sometimes a difference between the HTML render on Dodona and locally on your computer. Because the CSS properties of Dodona are inherited by your submission, your submission will get the Dodona lay-out when it isn’t actually intended to. A fix is in the works, but please note that the render is for informational purposes only. The judge only looks at the HTML and CSS and does not compare visually.