HowToDoTheThing: Cut Tags
Cut tags, content warnings, read more functionality...
There's nothing specifically for content warnings, but either the spoiler tag or summary tag could work to hide NSFW artwork or sensitive content.
[spoiler=clickable text]spoiler or content to be hidden[/spoiler]
I cannot show you an example here, but this is probably best used inline, for small sections of content, and can probably be used like a cut tag on LJ or Dreamwidth.
The spoiler or content being warned for goes between the tags, and when clicked, will display the hidden content.
[summary]summary of hidden content[/summary]
Summary tags are probably best for large chunks of content, or entire posts. The text between the tags is displayed as a post summary, and anything after the closed tag is hidden.
[open=clickable text]content to be hidden[/open]
Simple, non-specific, 'read more' type tag.