There is a solution that works 
you can find the discussion here:
[language-server.tailwind-heex]
command = "tailwindcss-language-server"
args = ["--stdio"]
[[language]]
name = "elixir"
language-id = "phoenix-heex"
language-servers = ["tailwind-heex", "elixir-ls"]
auto-format = true
[[language]]
name = "heex"
language-id = "phoenix-heex"
language-servers = ["tailwind-heex", "elixir-ls"]
auto-format = true
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