stat the word count for a given stackexchange user
Reviewing your T-SQL script I see that you have a cursor to iterate over Posts from a specific owner. For each body you use a neat trick to determine the word count of the body and use an extra variable to keep the running sum. You use FORMATMESSAGE to shape the data in each row.
Do know that the neat trick for the word count doesn't compensate for cases where two or more spaces are used, or where words are not separated by spaces but by other characters, like line-breaks for example. I'll ignore the fact here that in case of Posts.Body not only the text is stored but also the HTML markup.
I find the script clear and well structured.
If you're looking for an alternative way to accomplish the same output (I ignore the formatting here) I can offer this SEDE query:
DECLARE @UserId int = ##UserId##  -- SDE parameter syntax

SELECT Id
     , LEN(Body ) - LEN(replace(Body , ' ', '')) [count]
     , SUM(LEN(Body ) - LEN(replace(Body , ' ', ''))) 
         OVER (ORDER BY id) [total count]
FROM posts 
WHERE posts.OwnerUserId = @UserId
ORDER BY Id

I have started with your base query as found in the CURSOR but instead added the third column with an OVER( ORDER BY id) clause which enables the calculation of running totals over sets.  I'm a firm believer that when there is a feature available in the language it should be the preferred option, instead of running your own version in script. There is also a higher chance this gets better optimized by the query plan, if not today then possible in the future. Looking at wall-time and the query plan for your version and mine seems to support my gut feeling.
If you want to address the neat trick there are different ways to solve that, for example by evaluation the answers on  Removing repeated duplicated characters but all of those have pros and cons and without confirmation if this is an area you want to have addressed I assume for now you're fine with your method.