"de" is an undefined article which is whenever possible used instead of "des", indeed avoir "des beaux yeux" is grammatically correct and is the exact same thing as "de beaux yeux", it's just shorter and it sounds better. However, articles cannot be omitted in french, those are even here to change the meanings and form of a lot of verbs, like "coucher" (to put sb to bed or to lie down) becomes a pronoun when you say "un coucher de soleil" (a sunset)
@Bryanxu Yes that's a good comparaison. In English you would say "you have some beautiful eyes" and it would be compared to "tu as de beaux yeux", however in french this is more accurate as "de" depending on the context can also be a partitive article (de, d', du, de la) which is used whenever it's impossible to enumerate the object quantity such as "de l'eau", "de la nourriture" and so on. In your context you can enumerate the eyes so it's better to think "de" as an undefined article in contracted form.
in your other example you can say "Tu as des doux cheveux" but it's better to say "tu as de doux cheveux" (or "tu as des cheveux doux") as it sounds better and as you can't really enumerate how many cheveux you're talking about 😄
hopefully this is not too confusing, french can be permissive for conversational fluency but it's very easy to make grammatical mistakes, there are a lot of rules and details that makes this language so "poet" friendly, it is very informative especially in the written form and even if the majority of people including me have no clue of some rules or etymology from Latin, everything makes sense when you know about those!
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