PS. While debugging the project I can see that both the Text and the TextXHtml properties contains Markup language, even though the documentation specifies that only the later should. Could this somehow be related?

if you don't need spell checking straight off the ajaxtoolkit htmleditor works well!

Add this as the KeyPress client side event handler:

function HtmlEditorKeyPress(oEditor, keyCode, oEvent) {
    //Call update on each keypress to keep hidden field in sync with display.
    oEditor._update();
}

The problem is that in non-IE browsers the internal form variable is not updated as you type but only when focus is lost meaning that the old value can be set to the server if the blur event does not fire. The event handler above calls the internal function that does this syncing. Problem solved!

I am still not able to get value after postback on Chrome (working on IE). I tried following steps:

I created keypress event using below code.

<ClientSideEvents KeyPress="Editor1_KeyPress"></ClientSideEvents>

Next created below function 

function Editor1_KeyPress(oEditor, keyCode, oEvent) {

    //Call update on each keypress to keep hidden field in sync with display.

    oEditor._update();
}

When I type something in editor my control successfully enter the method and executes  "oEditor._update();" without any error.

Now, when I click on a button on page to make postback, I looses value of Editor