我如何知道何时使用CSS，ID或HTML5来使用CSS在HTML中进行布局？(How do I know when to use a Class, an ID or HTML5 to make layouts in HTML using CSS? [closed])

 ! 不是一个否定功能，它是一个宏。 
 
 请参阅elixir中的代码， lib/elixir/lib/kernel.ex: 1552 ， 
 
 defmacro !value

  defmacro !{:!, _, [value]} do
    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> false
          _ -> true
        end
      end
    )
  end

  defmacro !value do
    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> true
          _ -> false
        end
      end
    )
  end
 
 ! 宏扩展是一个case statement ，这是不允许在灵药卫士。 
 
 而不是一个erlang函数： 
 
  @spec not true :: false
  @spec not false :: true
  def not value do
    :erlang.not(value)
  end

! is not a negation function, it is a macro.
 
See the code in elixir, lib/elixir/lib/kernel.ex: 1552,
 
 defmacro !value

  defmacro !{:!, _, [value]} do
    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> false
          _ -> true
        end
      end
    )
  end

  defmacro !value do
    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> true
          _ -> false
        end
      end
    )
  end
 
The ! macro expanded is a case statement, which is not allowed in elixir guards.
 
And not is a erlang function:
 
  @spec not true :: false
  @spec not false :: true
  def not value do
    :erlang.not(value)
  end