In Microsoft SharePoint lists and libraries, calculated fields that are running in the classic UI mode can be used to display results that are based on formulas that resemble those in Microsoft Excel. This is a long-standing capability that 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 block the execution of custom markup in calculated fields in SharePoint Online from June 13, 2017 onwards. The June 2017 Public Update (PU) and later PUs will make blocking a configurable option for on-premises use in SharePoint Server 2016 and SharePoint Server 2013.
We are changing calculated fields to block the execution of custom markup. This mean that calculated fields that contain any unsupported markup will not display correctly.
This change went into effect on June 13, 2017. Administrators can request an extension through September 10, 2017 at the latest. During this extension, execution of custom markup in calculated fields will not be blocked. This request can be submitted through Microsoft Support. However, beginning September 10, 2017, all unsupported markup will be ignored.
SharePoint Server 2013 and SharePoint Server 2016
The June 2017 PU and subsequent PUs will include a new web application setting that's called CustomMarkupInCalculatedFieldDisabled. This setting lets an on-premises administrator determine whether execution of custom markup in calculated fields in a given web application is blocked.
The following is the default behavior for existing and new web applications at the time of the update installation.
Web app type
Newly created web app after the update is installed
Block execution of custom markup in calculated fields
Existing web app after the update is installed
Allow execution of custom markup in calculated fields
The following are some useful links to JSLink and CustomAction documentation:
Video: Client Side Rendering