HTL Spec was updated in June 2018 to support variable setting via data-sly-set (https://github.com/adobe/htl-spec/blob/1.4/SPECIFICATION.md#2212-set) and available in AEM 6.5 (https://docs.adobe.com/content/help/en/experience-manager-65/release-notes/sites.html), AEM 6.3 SP3 and AEM 6.4 SP1 (https://github.com/adobe/htl-spec).

This potentially means we should now recommend the usage of data-sly-set when possible.

We should probably also mention that setting up variables by abusing the data-sly-test.identifier pattern for cases other than test blocks is to be avoided when possible.(https://github.com/Netcentric/aem-htl-style-guide#4.5).