My expertise lies in SQL Server and not in HTML authoring. It took me over ten years, but now I have a style sheet which I use in some articles. (I adapt the articles to the style sheet as I have reason to revise them.)
Possibly the style sheet uses too many colours and fonts, but here is a short explanation of the most common things you will see:
Builtins – text with red/brown bold text refers to things that are built-in to the environment. That is, data types, built-in functions, system views etc in SQL Server. I also use it for methods, properties etc in .NET Framework when I talk about .NET.
User-defined names – text with this dark-green bold text refers to stored procedures, tables, columns etc that I introduce myself in the text. As with built-ins, it can also apply to user-defined names in .NET or other environments outside SQL Server.
Data values – oblique text in light green refers to data in tables etc.
File paths – this blue/purple text is for file names and file paths.
XML elements – this monospaced font appears in a few places where I talk about XML elements and attributes.
KEYWORDS – SQL keywords are always in all uppercase, but with a smaller font-size than normal uppercase. You may note that in difference to many authors, I always write the built-in types in lowercase, using the builtin style, since this is how the types appear in sys.types.
Plan operators – Operators and similar items that appears in a query plan appears in the same colours as builtins, but are not in bold face.
keywords – For keywords in .NET languages I used this monospaced font, since keywords in .NET are always lowercase. Note how ever that this is also the general code font, so it is not reserved for .NET keywords.
I cannot vouch for that I use these styles consistently, but I do have a Perl script to help me to apply many of the automatically.
By the way, if you don't like the font you see the articles in, you can change the style sheet in your browser – I don't set any font for the article bodies myself. (With one exception: the one for XML elements.)