There's been a lot of controversy over the future (and current) use of Flash and the adoption of Silverlight. Should I continue to use it if I am already or should I use something else, like HTML5?
You can use Flash / Silverlight, but I feel you provide some alternative means to the content if at all possible.
So long as your layers don't wreck the layers underneath them, add it all on and serve your users as best your budget allows.
I think this will boil down to personal opinion but I would say no.
No because your cutting out anything running Apple's iOS and while that may not be a massive problem there is no point starting at a disadvantage. I also find such plugins slightly annoying on a normal browser and have flash blocker and such installed and they are only enabled if I approve it.
I would say only use plug ins if you have a geniune need. Rather than for the sake of it.
It all depends on your target audience. Will the people you are building your site for expect Flash or Silverlight? Will they be put off if it is there? I would suggest using customer focused decision making on questions like this rather than technical. Get to know your customers and plan accordingly.
Personally, I have a Flash blocker installed because I don't much care for Flash or Silverlight. I'm also not a professional web developer but I feel that the safe answer is probably "it depends on who your target audience is". At this point it seems that HTML 5 has a bright future and the potential to reach a broader range of devices- but Flash does have a large installed base and can be used for lots of things right now without worrying about how modern the user's browser is.
That being said, I did notice an interesting use that exploits Silverlight's connection to the .NET framework. trypython.org uses Silverlight to deliver the IronPython into the client's browser which allows for all sorts of neat shenanigans.
If the audience your website is targeted to doesn't use IE, then no, you should not use Flash or Silverlight.
If the audience your website is targeted to does use IE, and if you need to do something that only Flash or Silverlight can do (deliver video, let's say) then you don't have any alternatives that are as easy to deal with, or with as large a userbase.
If what you want to do with Flash or Silverlight is something that shouldn't be done with them in the first place (a navigation, let's say) then I have nothing left to say to you. Good day, sir.
The big controversy over Flash or Silverlight is about lack of smartphone(notably iPhone) support, instability/vulnerability of plugins and bad accessibility of plugin-based websites.
If you want to target the iPhone, then definitively don't use plugins at all.
Otherwise, think about which features you need:
- If the only way to do it is with plugins, then use plugins.
- If it is available in newer standards(HTML5/CSS3), but some browsers don't support it(IE6, for instance), but graceful degradation is possible, then build it in those standards and gracefully degradate.
- If graceful degradation is not possible, is it acceptable to have two versions of your application? If so, then you could have a HTML5 version and a plugin version. This is what YouTube does.
- If it's not acceptable to have two versions, then who is your average user? How many users have "modern" browsers vs. how many users have plugins? How will the situation be in the next few years? Based on the answers, either pick HTML5 or plugins.
However, remember that there is no single solution to the question "HTML5 or plugins?". Different websites have different needs.