Setting the default botton that needs to be pushed when the Enter key is pressed into an ASPX page, can be implemented easly in Asp.Net.
Imagine that your page holds 2 controls:

• a Asp.net TextBox named tb1
• a Asp.net Button named btn1
  

To instruct the page, to point the default button  on btn1, you should use:

Setting the focus on tb1 can be accomplished by:

As you can see, you have to use control's ClientID and not  the control's ID.

But what happens if you want to set the Page's DefaultButton property to a login button contained in a Login form control? Consider surrounding the <asp:Login /> control with a simple asp.net Panel:

...and  set the DefaultButton property using the syntax provided in the example.

If you need to add codes through Expression Bulders, you' ll notice that are different ways to do that. Depending on what you need to write into your html rendered page, you' ll need to use different annotations. Let's see:

Write a string Resource

Call a DataBind

Write a string through Response.Write

<%= DateTime.Now %>

The problem comes out when you try to use the Response.Write method to manipulate an asp.net server control's propery. The following annotations won't work:

 <asp:Label id="lbl1" runat="server" Text="<%= DateTime.Now %>"/>

The best solution to this problem is ito implement CodeExpressionBuilder posted in InfinitiesLoop's blog. Here's a quick howto:

1- Create a class in your Web Application Project, and paste:

2- Register the new Expression Builder into your Web.Config

And now you can call your code in server control's properties using the following syntax:

You just need to provide a simple Message box that asks for confirmation when a button is clickked? There are several ways to accomplish this, but if you don't need a complicated solution consider in adding a small javascript to the Asp.Net control on the OnClientClick attribute:

A message box will be prompted when users clicks this Imagebutton, which will raise the postback only if the 'yes' button is pressed. Cool..

Is not really intuitive how can you add a reference to a css stylesheet at runtime in ASP.NET in the HTML head section: you need to call the page's ClientScript manager and register a Javascript script block defining your script code as a link tag. Remember that RegisterCssReference method will only work if you call it before the page's OnRender event method, so use it in OnLoad and OnPreRender page's events.

Immagine a scenario where you need to handle how a browser should open different urls into your site. You need to open every link into a new browser's window except links to documents, to avoid opening a blank window.
Infact, using the attribute 'target' into an anchor: 

will open first a new window, and then let you download the file.As you can see  this is not a good solution for this scenario.
Here you can find a simple Javascript that will resolve this kind of problem