Explain that .csp files are HTML files.

They are “C++ Server Pages” from Drogon.

init.d/programming/lsp.el

@@ -52,6 +52,7 @@
               (add-to-list 'lsp-clients-clangd-args "--background-index" t))
             ;; Mark lsp-clients-clangd-args as safe to override.
             (put 'lsp-clients-clangd-args 'safe-local-variable #'consp)
+            (add-to-list 'lsp-language-id-configuration '(".*\\.csp$" . "html"))
 
             (defun my/lsp-init ()
               "Change some settings after entering lsp-mode."

init.d/text/web.el

@@ -38,7 +38,8 @@
                  (replace-match "<p>\\1</p>"))))
   :mode (("\\.[ps]?html?$" . web-mode)
          ("\\.tmpl$" . web-mode)        ; Gitea templates.
-         ("\\.php$" . web-mode))
+         ("\\.php$" . web-mode)
+         ("\\.csp$" . web-mode))        ; Drogon templates (C++ Server Pages).
   :hook (web-mode  . (lambda () (set-fill-column 120)))
   :bind ("C-c C-M-h" . my/html-surround-with-tag))