Safari: swap alt modifier with cmd
You can use Karabiner with a private.xml such as this:

&lt;?xml version="1.0"?&gt;
&lt;root&gt;
  &lt;appdef&gt;
    &lt;appname&gt;SAFARI&lt;/appname&gt;
    &lt;equal&gt;com.apple.Safari&lt;/equal&gt;
  &lt;/appdef&gt;
  &lt;item&gt;
    &lt;name&gt;Swap ⌥↩︎ and ⌘↩︎&lt;/name&gt;
    &lt;identifier&gt;SwapAltEnterAndCmdEnter&lt;/identifier&gt;
    &lt;only&gt;SAFARI&lt;/only&gt;
    &lt;autogen&gt;
      __KeyToKey__
      KeyCode::RETURN, VK_COMMAND | ModifierFlag::NONE,
      KeyCode::RETURN, ModifierFlag::OPTION_L
    &lt;/autogen&gt;
    &lt;autogen&gt;
      __KeyToKey__
      KeyCode::RETURN, VK_OPTION | ModifierFlag::NONE,
      KeyCode::RETURN, ModifierFlag::COMMAND_L
    &lt;/autogen&gt;
  &lt;/item&gt;
&lt;/root&gt;


You can find the manual here to know how to edit the private.xml file and update your configuration.

You have to check the small checkbox of the newly defined rule to make it work.