Possibly the smallest stand-alone web page that is useful is:

<html><body>
<textarea rows=20 cols=40 ondblclick="A.value=eval(this.value);"></textarea>
<input id="A"> </body></html>

Type some figures such as 12.34 +56.78*3 and double-click in the typing area.
- Try It
Put the figures in columns, it makes no difference (but don't omit the plus signs).
Add spaces as required (but not within numbers).

It may not look as neat as a calculator, but you can check and alter you workings, or cut and paste them to a file.

Or for something a bit more ambitious, type in (or copy and paste):
n=20
m=n
while(--m>0)n*=m

- This calculates the Factorial of 20 (20! = 20*19*18...*1).
(The --m bit just subtracts one from m each time.
And n*=m is the same as n = n*m.)
(Put a semicolon after a statement if you want to put more on the same line.)

The web page itself can be saved to a local file by using the browser 'File' and 'Save as' facility,
or by using an editor such as Notepad to save the above text, in a file called, say, MicroCalc.html
To run it, locate the file using Windows Explorer and double click on it,
or locate the file using a browser.

MaxiCalc help