I'm a new comer to the asp.net world. I hear a lot about asp.net mvc and it's advantage over webforms about the ability to customize the markup and css. I also heard that asp.net is much easier to learn than asp.net mvc so I decided to go for asp.net and webforms. My question is: what's the level of customization a web developer/designer can get with webforms concerning the markup and css?
Standard ASP.NET WebForms uses server controls that generate the markup for you, so the level of customization is limited to what the controls you are using provide. There are techniques that allow you to override what is rendered by the controls and thus customize the markup and also write your own controls but it requires some coding. It is possible to achieve almost complete customization of the markup but it IMHO requires more efforts than a web developer should need to put into something like this.
While it is definitely possible to have a SEO friendly, unit-testable, maintanable, standards compliant application using classic ASP.NET WebForms, the efforts it requires will be significant compared to ASP.NET MVC. But if you don't care about those things you will be able to pretty quickly develop web applications.
ASP.NET comes with a set of built in user controls - things like text areas, buttons etc. These mimic winforms in how they are supposed to work (events etc), however this is a rather leaky abstraction (you must always remember you are working with HTML and HTTP).
The user controls allow reuse and when a page is built they emit HTML - you have little control over the emitted HTML (unless you override the rendering, which kinda defeats the point), hence the perception that they are harder to customize. It is not easy to get right either and requires more work than I think is worth.
There are also different compromises in the way pages are rendered out (ids for example end up as a long string of concatenated container control names) which make MVC a better choice if you are looking for control over your HTML.
Microsoft suggests that you pick the technology based on your needs: While ASP.NET offers rich controls and produces quick results without great control over the markup (as mentioned: it can be done but somehow beats the idea behind ASP.NET and creates a lot of additional code), it suffers from the flaws mentioned by the other posters.
In MVC, there is a limited set of "out-of-the-box" controls and you'll have to code more on your own (including clientside JavaScript) but you do have more control over the rendered markup of your controls. In addition to that, your project will generally have a clean separation of concerns which benefits (unit) testing and maintainance.
Another aspect that hasn't been mentioned yet: In ASP.NET a page undergoes the so-called "ASP.NET Lifecycle" every time the client communicates with the server. The Lifecycle consists of several events that are fired in a special (and sometimes confusing) order. Handling those event in the right order in complex web applications is one of the biggest difficulties in ASP.NET and often leads beginners to forfeit. In MVC you don't have to deal with that kind of problem. Therefore I strongly recommend that you take a look at the ASP.NET architecture before you start to code. Here is a very basic start: http://www.asp.net/learn/videos/video-6558.aspx
Personally, I started with WebForms and am now moving to MVC after I worked with the similar MVVM pattern in Silverlight and WPF for my bachelor thesis. This kind of did it for me so that I now understand the benefits and ideas behind MVC a lot better. Once you are used to WebForms, switching won't be that easy though.
You can have as much customisation as you like in the html output! You can customise everything in web forms. However, with customisation brings time, effort and room for error. All of these things is what web forms is trying to save you.
However, since you are just starting out I wouldn't worry. Just make your web forms how you want and forget the customisation of output (it is much better with ASP.NET 4 anyway). In a few years when you are more experienced then worry.
If you were going to customise everything then you should have gone with ASP.NET MVC - it is one of its main arguments. But there is nothing wrong with web forms. Particularly if you are beginning with asp.net in general I'd say it is better.