If you want to do that, you're probably taking the wrong approach to spoilers. I'm having trouble distilling this into words, so let me give you two examples:
Need help with the final jump in Portal. portal spoiler
SPOILER ALERT ZOMG!
I've reached the end of Portal and I'm having trouble with the final jump of the game, which should trigger the ending cutscene. I can't be more specific without spoiling everything and I haven't put in 300 characters yet, so let me add some generic filler about how this game is totally awesome and you should buy it yesterday.
Phew, here goes my real question! Do not hover with your mouse if you haven't played through yet, because the ending is so totally awesome, if you haven't seen it yet I envy you because it's so absurdly awesomely mindblowing.
I can't reach the anger core. I try to place a portal at the very top, but I always fall a bit short of grabbing the personality core, no matter how hard I tap E.
How can I do this?
There are several wrong things with the above, even when you remove my touch of humor above.
The question title just can't be googled for. Stack Exchange is supposed to be write once, read many. Obfuscating the title to skirm out spoilers seriously hampers this. People don't google for "how can I do the final jump in portal", people google for "portal ending boss-fight walkthrough", or at most "portal ending third core."
The meat of the question is padded by filler and locked behind JS trickery. If JS is disabled, spoiler blockquotes don't work. If JS is enabled but you can't hover (think smartphones), spoiler blockquotes are a PITA to read at all. Additionally, tooltips, chat and (most importantly) Google do not support spoiler markup at all, and with search engines it doesn't matter if you count letters or paragraphs.
The spoiler tag. (Bonus point if the question has "[SPOILERS]" in the title.) The spoiler tag is a poor idea, mainly because it's so generic. It's trivial to have something that is much more specific and is not a meta-tag, such as plot, ending or boss-fight.
"Wait a second, Gaming does have a spoiler tag!" Yeah, it does. Since we have a non-written policy of writing good titles even if they spoil, I proposed to use that tag to mark spoilery titles so that you can "hide them" easily. Even in this way, however, this isn't really a good idea; it'd be much better to place topics you don't want to be spoiled about in your ignore list.
Compare with:
I can't reach the anger core. I try to place a portal at the very top, but I always fall a bit short of grabbing the personality core, no matter how hard I tap E.
How can I do this?
The title is clear, googleable, and meaningless if you haven't finished the game (so it cannot really spoil!). The question text goes straight to the matter. The tagging is pertinent to the question. I don't know about you, but I like the second approach much better.
Be aware that most of this actually applies to answers as well. If you have a paragraph discussing the spoilery matter that is at the heart of the question, you shouldn't put it in a spoiler tag; it's redundant. You should limit the spoiler section in answer for spoilers that are unnecessary and tangential to the answer. For example:
To reach for that core, you must fling. Place the first portal at the top, place the second on your feet; while you go through, aim down and shoot the second portal on your feet again; when you go through again, aim forward and you'll be in grabbing range.
Once you have the core, you'll need to destroy it like you have done with the other two cores: place a portal near the incinerator, go in the "room" on the left, place the second portal there, press the button, go through and drop the core in there. You win!