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Hi Edward,
In Microsoft SharePoint lists and libraries, calculated fields that are running in the classic UI mode can be used to display results based on Excel-like formulas. This is a long standing capability, and is documented in Calculate data in lists or libraries.
Some users have added HTML markup or script elements to calculated fields. This is an undocumented use of the feature, and we will begin blocking execution of custom markup in calculated fields in SharePoint Online from June 13, 2017 onwards.
We are changing calculated fields to escape special characters. In some circumstances, this means that calculated fields that contain any unsupported markup will not display any value. Instead, they'll be blank in the list view.
This change will take effect on June 13, 2017. Administrators can request an extension through September 10, 2017 at the latest. During this time, special characters will not be escaped in calculated fields. This request can be submitted through Microsoft Support. However, beginning September 10, 2017, all unsupported markup will be ignored.
For more information, please refer to the following KB.
Handling HTML markup in SharePoint calculated fields
Thanks,
Ran