MLS have sent through some advice to resolve some of the HTML5 problems and improve your experience for all .net hosted customers please follow the below guidance:
The following articles can be used to resolve all loading/speed issues with HTML5 Management.
https://www.microlib.co.uk/myMLS/KnowledgeBaseArticle.aspx?ArticleID=1027 - Proxy Exceptions
There is an issue with compatibility sites being added within
Internet Explorer we would need the *microlibrarian.net address removing from
1. Access ‘Tools’ or the ‘Cog Wheel Icon’ at the top right using
Internet Explorer 11.
2. Open the setting labelled ‘Compatibility View Settings’.
3. Ensure that the ‘*.microlibrarian.net’ address and any other
‘microlibrarian’ address has been removed.
4. Re-load the management page within Internet Explorer and the content should now display fine once more.
Kind regards, Andy Macfarlane