While there's undoubtedly a learning curve and the syntax can take some getting used to, HTML and CSS are not coding languages. Participants won't be asked to use coding concepts like loops and variables. HTML is concerned with how documents or pages are structured while CSS is concerned with how they look. Once participants get used to the strange looking tags in HTML and get past the curly brackets and semi-colons of CSS they'll see that it's just a vocabulary to get comfortable with.
While the internet accounts for many sites and tutorials including W3C Schools, Mozilla Developer Network and CSS Tricks, the amount of information available can also feel overwhelming. As such, we recommend Jon Duckett's book on HTML and CSS. Though published in 2011 it is still the best designed desk reference which doesn't feel like a technical manual.