If you play a YouTube video, look at the URL - it will probably look something like this: http://www.youtube.com/watch?v=L5Y4qzc_JTg.
The video ID is the list of letters and numbers after `v=`, in this case `L5Y4qzc_JTg`.
Vixy YouTube Embed uses the latest version of the YouTube Player, named AS3. This means that some options are no longer available (however, some new ones are). See this document for a complete list of the supported parameter for each player type.
The border option is no longer available with the YouTube player. However, you can use the `style` option to mimic it. Simply add a style of border: 10px solid #b1b1b1.
This has been reported by a number of users - for some reason some people are unable to access the YouTube API, which is used by this plug-in to determine the video type and whether it's valid or not. If you are one of them then simply go to the Options screen within the YouTube Embed administration menu and, near the bottom, there is an option to stop reporting API errors.

If you switch off API errors then the plug in will simply assume all IDs are valid and work out the type from the ID length (which is not set, so if this changes in future it may cause problems!).

I have requested further information on why these errors may be occurring from the YouTube forum but, as yet, I've had no response. You can track the request here.
Code created for previous versions of the plug in should still work - however, the functions will not be documented to deter people from using them in the future.
It should be, yes, but with one exception. If you have upgraded from version 2.0.1 or before and used the `style` parameter then it may cause some problems. This is because the `style` parameter now affects the CSS of the video. Before, it added a DIV "wrapper" around the video and applied the styles to this. Therefore, if you used the `style` parameter to centre a video this probably doesn't now work. However, you can do this now by using the `template` parameter instead - please read the section on that for further help.

However, with the change to the way the `style` works you can now apply more direct styling to the video - e.g. adding a border.
In all cases, yes. However, it depends on which options you choose as to which DOCTYPE it will validate to.

IFRAME does not work with XHTML so will only validate as XHTML transitional (this includes if you use IFRAME as the fallback to EmbedPlus).

If you include Metadata then you must use the XHTML Strict + RDFa DOCTYPE - in this case it validates.

If you don't include Metadata then it will also validate as HTML5 compliant.

The W3C Markup Validation Service was used to test the above.

This spreadsheet shows which embed types were tested and to where they validated (or didn't).
It uses standard OBJECT and IFRAME coding - this, along with the above standards compliance, means that the output should work on most browsers.
If you go into the Profile screen in Administration there is a Window Mode option. This defines how Flash output interacts with any other around it. Window is the default and gives good performance but may cause overlapping. If overlapping is causing an issue try Opaque.
If you find no video is showing but there's no error either check the source code of the page. Can you find the text "<!--YouTube Error: bad URL entered-->" in the page? If so, you have Jetpack installed and that is displaying the video instead.

There are two ways around this. First, you could disable all the shortcodes in Jetpack - Jetpack allows you to deactivate certain "modules" of the plug in and, if you can live without the other extra shortcodes, deactivating the shortcodes part will restore functionality back to YouTube Embed.

Alternatively, you can use a secondary shortcode - [youtube_video]. Use this instead of [youtube] and YouTube Embed will render the results without a problem. This second suggestion will work for any plug in that may use the same shortcode.
WordPress will, by default, convert YouTube URLs to videos using a very basic default configuration. Earlier versions of this plug in had an option to override this so this plug in would output the results instead (giving you full control of the output). Unfortunately, it stopped working and I do not have enough knowledge on the required WordPress code to work out how to fix it. Rather than leave broken code in place I have removed it.

If you wish to have full control over your YouTube output I would suggest placing all YouTube URLs within [youtube] shortcodes.
Due to a housekeeping limitation in WordPress cache entries, which are stored in the OPTIONS table, may get left behind. There are two solutions to clearing this. First of all, in the Options menu in YouTube Embed administration, there is an option in the performance section named Clear Cache. Simply tick this box and click on the "Save Settings" button to clear out any cache. However, the best option is to install the plug in Artiss Transient Cleaner, which will housekeep the OPTIONS table automatically.
If you display the video responsively then it doesn't work with Turn Off The Lights. Switch this option off and it will.

I've raised this with the developer of Turn Off The Lights and he is going to implement a change in the future to improve compatibility with responsive video.
Your version of Flash is out-of-date and needs updating.
Unfortunately, this is a restriction that has been put in place by Apple.
This is a known issue with the YouTube player and is with Google to fix.
There is no way to specify this - YouTube makes the decision on which version to play depending on a number of factors. The old version of their player had an "HD" option but that too didn't guarantee HD playback.
Before reporting it please bear in mind that this plug in uses the standard YouTube API. Adding extra functionality to the player itself is not possible and there are known issues with it. I would also recommend performing a Google search for your issue first, as this will often resolve a lot of queries.
It has been syntax checked as PHP 4. However, this does not guarantee PHP 4 compatibility and the minimum for WordPress is now PHP 5.2.4.

Although I attempt to keep with PHP 4 compatibility there are no guarantees of this.