It's not really a struggle, but when all you have is a string, how do you enforce certain rules and requirements?

E.g. classMap https://lit.dev/docs/templates/directives/#classmap

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The classMap must be the only expression in the class attribute, but it can be combined with static values

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So now you have to figure out which attribute this is called from, whether this particular call is allowed in this attribute etc.

So what they do is they parse (with regexes) their "close to HTML" code into a structure not dissimilar to React's, figure all this stuff out, reassemble actual HTML and dump it to the DOM