Safari needs to load the xml file to preview HTML5 Slideshow. If Safari fails to clear the caches of the previous HTML5 slideshow, the newly opened preview window will show the following message. All you want to do is to clear the caches. You can temporary solve the HTML5 slideshow preview problem by exiting Safari and reopenning the preview window. Or you may need to take the following steps to solve it permanently.
Step 1. Press "Command" and "," to get the following window.

Step 2. Check the "Show Develop memu in menu bar".
Step 3. Check "Disable Caches".
As the error message shows, safari failed to preview HTML5 slideshow because it declined the request of XML and Http files. If your Mac is 10.8.2 or later, please check both "Disable Caches" and "Disable Local File Restrictions" to solve this problem. If the problem insist, please take the following steps.
Step 1. Exit Safari and open "Terminal".

Step 2. Add "open -a '/Applications/Safari.app' --args --disable-web-security" to the end of the content.
Step 3. Press "enter". Safari will open again and it will allow to load the local XML and Http files.
Because the default settings in Chrome disallow XML and Http file request, so you are unable to open your HTML5 slideshow in Chrome and Opera unless you change the default settings.
Step 1. Right click your Chrome shortcut, choose “Properties”, and find “Target”.
The exmaple of the original content in “Target” is "C:\Program Files\Google\Chrome\Application\chrome.exe", as shown in the following figure.

Step 2. Add " --allow-file-access-from-files" to the end of the content.
In the example above, the original content is "C:\ProgramFiles\Google\Chrome\Application\chrome.exe", you need to change it to "C:\ProgramFiles\Google\Chrome\Application\chrome.exe" --allow-file-access-from-files.
Step 3. Save your change.
The default settings in Opera disallow XML and Http file request too, so in order to preview HTML5 slideshow on Opera, you need to change the default settings like below.
Step 1. Open Opera, and enter “opera:config” in the address. You will get a frame like below.

Step 2. Find “Use Prefs”, click it.
Step 3. Find “Allow File XMLHttpRequest”, and put a check mark as the picture below.

Step 4. Save your change.