You could potentially learn HTML and CSS at the same time. HTML is much simpler than CSS. if it were to be measured in units, HTML would be about 2 units and CSS would be 7 units. JS on the otherhand is about 13 units. JQuery would be 4 units.
but dont worry about learning something completely, just make sure you cover your fundamentals first but make a note to revisit concepts you are unfamiliar with later to cover your blind spots.
I for example knew more about JS than CSS until I recently decided to revisit CSS
I do not know in which sense you use 'completely', but it would help you a lot to have had already covered the html fundamentale when diving into CSS. The html course is short and not at all difficult and you lern there about the objects and attributes which you will be eventually manipulating through your CSS.