I'm surprised no one mentioned the HTML entities  
and  
which produce horizontal white space equivalent to the characters n and m, respectively. If you want to accumulate horizontal white space quickly, those are more efficient than
.
 
 
Along with <space>
and  
, these are the five entities HTML provides for horizontal white space.
Note that except for
, all entities allow breaking. Whatever text surrounds them will wrap to a new line if it would otherwise extend beyond the container boundary. With
it would wrap to a new line as a block even if the text before
could fit on the previous line.
Depending on your use case, that may be desired or undesired. For me, unless I'm dealing with things like names (John
Doe), addresses or references (see eq.
5), breaking as a block is usually undesired.