Wow, I think I got me a new hobby 
– To nitpick, searching a symbol for the “space bar” on the keyboard is one more, new, interesting quest, as it is certainly not equivalent to a symbol for the “space character” in a string …
From the list which reads kind-of “official”, given over there:
- U+00B7 “
·” Middle dot - U+237D “
⍽” Shouldered open box - U+2420 “
␠” Symbol for space - U+2422 “
␢” Blank symbol - U+2423 “
␣” Open box
(and I would add the symbol that looks like U+25B3 “△” WHITE UP-POINTING TRIANGLE, used in ISO 6093:1985 for example, if that’s not GREEK CAPITAL LETTER DELTA), I have to say:
I like
- “
␣” OPEN BOX the best, and then - “
△”, this triangle-thingy, and then - “
·” MIDDLE DOT on third place.
(The BLANK SYMBOL reminds me of punched cards, the SYMBOL FOR SPACE treats poor U+0032 as if it were a control character, the SHOULDERED OPEN BOX is completely new to me, and from the description “graphics for keyboard symbol for no break space” it is not what we look for anyway, and finally MIDDLE DOT is at least in ISO 8859-1).
But what really bothers me is the fact that U+2423 OPEN BOX is, on the system I’m sitting at right now, “only” available in the fonts:
- DejaVu Sans
- DejaVu Sans Mono
- DejaVu Serif
- FreeMono
- FreeSans
- FreeSerif
- Linux Biolinum G
- Linux Libertine Display G
- Linux Libertine G
- Lucida Sans Unicode
- Meiryo
- Meiryo UI
- MS Gothic
- MS Mincho
- MS PGothic
- MS PMincho
- MS UI Gothic
- Segoe UI Symbol
but not in my preferred monospace font in the browser, Cousine, and that I therefore have no idea where the “␣” I’m looking at is actually coming from, but I think I can see that this one has the wrong metrics and does not match Cousine’s monospace pitch …
Eeeeehm, what was the question again???
[ Edit:
I prefer the one you used, as the “true” space bracket is really tiny!
In this case, how about some nice, big “⊔” U+2294 SQUARE CUP? (Doesn’t match either here, though!)
12345678
⊔⊔⊔⊔code block
⎵⎵⎵⎵code block
␣␣␣␣code block
····code block (here's MIDDLE DOT for comparison!)
12345678
]
[ Edit 2:
Switching to DejaVu Sans Mono as the browser’s monospace font solves the font metrics problem, as you can see in this screenshot: MIDDLE DOT and OPEN BOX match the font’s pitch, but SQUARE CUP and let alone BOTTOM SQUARE BRACKET don’t:
]