* lisp/textmodes/css-mode.el (css--html-tags): New variable holding a
list of HTML tags for completion.
(css--nested-selectors-allowed): New variable for determining whether
nested selectors are allowed in the current mode.
(css--complete-selector): New function for completing part of a CSS
selector.
(css-completion-at-point): Support completion of selectors.
(scss-mode): Allow nested selectors.
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