Someone asked me to compare Silverlight / HTML development. I realized that the question can be answered in many ways:

Below is the high level comparison between a HTML /JavaScript client and Silverlight client and why silverlight was chosen over HTML / JavaScript client (based on type of users and major functionalities provided):

1. For end users

Browser compatibility

Silverlight is a plug-in and requires installation first. However, it does provides consistent look and feel across all browsers.

For HTML / DHTML, there is a need to tweak JavaScript for each of the browser supported. In fact, tags like <span> and <div> works differently on different browser / version. So, HTML works on most of the systems but also requires lot of efforts coding-wise to adhere to all standards/ browsers / versions.

Out of browser support

No support in HTML. Third party tools like  Google gears offers some functionalities but there are lots of issues around platform and accessibility.

Out of box support for out-of-browser support. provides features like drag and drop onto application surface.

Cut and copy paste in HTML

HTML is displayed in browser; which, in turn provides facilities for cut copy and paste.

Silverlight (specially 4) provides rich features for cut-copy-paste along with full control over what can be cut copy pasted by end users and .advanced features like visual tree printing.

Rich user experience

HTML can provide some rich experience by use of some JavaScript libraries like JQuery. However, extensive use of JavaScript combined with various versions of browsers and the supported JavaScript makes the solution cumbersome.

Silverlight is meant for RIA experience.

User data storage on client end

In HTML only small amount of data can be stored that too in cookies.

In Silverlight large data may be stored, that too in secure way. This increases the response time.

Post back

In HTML / JavaScript the post back can be stopped by use of AJAX. Extensive use of AJAX can be a bottleneck as browser stack is used for the calls. Both look and feel and data travel over network.                          

In Silverlight everything run the client side. Calls are made to server ONLY for data; which also reduces network traffic in long run.

2. For Developers

Coding effort

HTML / JavaScript can take considerable amount to code if features (requirements) are rich. For AJAX like interfaces; knowledge of third party kits like DOJO / Yahoo UI / JQuery is required which has steep learning curve. ASP .Net coding world revolves mostly along <table> tags for alignments whereas most popular tools provide <div> tags; which requires lots of tweaking.

AJAX calls can be a bottlenecks for performance, if the calls are many.

In Silverlight; coding is in C#, which is managed code. XAML is also very intuitive and Blend can be used to provide look and feel. Event handling is much clean than in JavaScript.

Provides for many clean patterns like MVVM and composable application. Each call to server is asynchronous in silverlight. AJAX is in built into silverlight. Threading can be done at the client side itself to provide for better responsiveness; etc.

Debugging

Debugging in HTML / JavaScript is difficult. As JavaScript is interpreted; there is NO compile time error handling.

Debugging in Silverlight is very helpful. As it is compiled; it provides rich features for both compile time and run time error handling.

Multi -targeting browsers

HTML / JavaScript have different rendering behaviours in different browsers / and their versions. JavaScript have to be written to sublime the differences in browser behaviours.

Silverlight works exactly the same in all browsers and works on almost all popular browser.

Multi-targeting desktop

No support in HTML / JavaScript

Silverlight is very close to WPF. Bot the platform may be easily targeted while maintaining the same source code.

Rich toolkit

HTML /JavaScript have limited toolkit as controls

Silverlight provides a rich set of controls including graphs, audio, video, layout, etc.

3. For Architects

Design Patterns

Silverlight provides for patterns like MVVM (MVC) and rich (fat)  client architecture. This segregates the "separation of concern" very clearly. Client (silverlight) does what it is expected to do and server does what it is expected of.

In HTML / JavaScript world most of the processing is done on the server side.

Extensibility

Silverlight provides great deal of extensibility as custom controls may be made. Extensibility is NOT restricted by browser but by the plug-in silverlight runs in.

HTML / JavaScript works in a certain way and extensibility is generally done on the server side rather than client end. Client side is restricted by the limitations of the browser.

Performance

Silverlight provides localized storage which may be used for cached data storage. this reduces the response time.

As processing can be done on client side itself; there is no need for server round trips. this decreases the round about time.

Look and feel of the application is downloaded ONLY initially, afterwards ONLY data is fetched form the server.

Security

Silverlight is compiled code downloaded as .XAP; As compared to HTML / JavaScript, it provides more secure sandboxed approach.

Cross - scripting is inherently prohibited in silverlight by default.

If proper guidelines are followed silverlight provides much robust security mechanism as against HTML / JavaScript world. For example; knowing server Address in obfuscated JavaScript is easier than a compressed compiled obfuscated silverlight .XAP file.

Some of these like (offline and Canvas support) will be available in HTML5. However, the timelines are not encouraging at all. According to Ian Hickson, editor of the HTML5 specification, the specification to reach the W3C Candidate Recommendation stage during 2012, and W3C Recommendation in the year 2022 or later. see http://en.wikipedia.org/wiki/HTML5 for details.

The above is MY opinion. I will love to hear yours; do let me know via comments.