No there's no reason you can't use classes everywhere for styling. Even if you think you'll only want to use a style once, there's no reason not to use a class for it. I can see why you would define "single-use" styles on the IDs but there isn't any advantage to doing that except for the higher specificity. I prefer to reserve IDs for other uses such as form labels and linking to anchor tags and keep all my styles defined by classes, for consistency.
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No there's no reason you can't use classes everywhere for styling. Even if you think you'll only want to use a style once, there's no reason not to use a class for it. I can see why you would define "single-use" styles on the IDs but there isn't any advantage to doing that except for the higher specificity. I prefer to reserve IDs for other uses such as form labels and linking to anchor tags and keep all my styles defined by classes, for consistency.