This snippet shows how you can detect CSS classes in html and display diagnostics about them if desired. See comments for credits (not me).

nvim-html-diagnostics-for-css-classes.lua

-- https://linktr.ee/TymekDev
-- Alright, I got the HTML -> classes -> diagnostics part. Now all that's left is:
-- 1. Parse CSS file(s) for selectors
-- 2. ~~Intersect~~ Get set difference of class names and selectors
-- 3. Run this stuff on save (?)

-- Anyone wants to take this home? <@216716701973217290>?

-- Demo: https://asciinema.org/a/VQEt82360Kugo0gkg33fVYe57

-- Code:
local query = [[(
  (attribute_name) @name (#eq? @name "class")
  (quoted_attribute_value
    (attribute_value) @classes)
)]]

local get_root = function(bufnr)
    local parser = vim.treesitter.get_parser(bufnr, 'html', {})
    local tree = parser:parse()[1]
    return tree:root()
end

local get_classes = function(bufnr)
    local ts_query = vim.treesitter.query.parse('html', query)
    local result = {}
    for id, node, _ in ts_query:iter_captures(get_root(bufnr), bufnr) do
        if ts_query.captures[id] == 'classes' then
            local start_row, start_col, _, _ = vim.treesitter.get_node_range(node)
            local text = vim.treesitter.get_node_text(node, bufnr)
            local offset = 0
            for class in string.gmatch(text, '[^%s]+') do
                table.insert(result, {
                    bufnr = bufnr,
                    lnum = start_row,
                    col = start_col + offset,
                    message = "Unused class '" .. class .. "'",
                })
                offset = offset + #class + 1
            end
        end
    end
    return result
end

local show_diagnostics = function(bufnr)
    local ns = vim.api.nvim_create_namespace 'HTML Classes Linter'
    vim.diagnostic.set(ns, bufnr, get_classes(bufnr))
end

local bufnr = 1
show_diagnostics(bufnr)