How do I put two spaces after every period in our HTML?

You might be able to use the JavaScript split method or regex depending on the scope of the text.

Here's the split method:

var el = document.getElementById("mydiv");
if (el){
    el.innerText = el.innerText.split(".").join(".\xA0 ");   
}

Test case:

Hello world.Insert spaces after the period.Using the split method.

Result:

Hello world. Insert spaces after the period. Using the split method.

As we all know, HTML collapses white space, but it only does this for display. The extra spaces are still there. So if the source material was created with two spaces after each period, then some of these substitution methods that are being suggested can be made to work reliably - search for "period-space-space" and replace it with something more suituble, like period-space- . Please note that you shouldn't use   because it can prevent proper wrapping at margins. (If you're using ragged right, the margin change won't be noticeable as long as you use the the nbsp BEFORE the space.)

You can also wrap each sentence in a span and use the :after selector to add a space and format it to be wide with "word-spacing". Or you can wrap the space between sentences itself in a span and style that directly.

I've written a javascript solution for blogger that does this on the fly, looks for period-space-space, and replaces it with a spanned, styled space that appears wider.

If however your original material doesn't include this sort of thing then you'll have to study up on sentence boundary detection algorithms (which are not so simple), and then modify one to also not trip over PHP code.