If you are creating a custom theme for Publii, you might want to list all posts with a specific tag on a dedicated page. Here's how to do it:

First, create an override folder as described here: https://getpublii.com/dev/theme-overrides/
For the Mercury theme, the folder would be named mercury-override.

Then, copy the config.json file from the original theme folder into the override folder and add the following:

"postTemplates": {
  "portfolio": "portfolio filter"
},

Replace the word "portfolio" with the tag you want to filter by. For example, if you have multiple posts tagged as 'Mallorca,' it would look like this:
"postTemplates": {
"mallorca": "mallorca filter"
},
Note the use of lowercase letters.

A detailed guide can be found here:
https://cv.domagic.site/posts/alternative-post-template-with-filter-for-publii-cms

Now duplicate the index.hbs file and move the copy to the previously created override folder. Rename the file to post-newname.hbs. In this example, the file should be named post-mallorca.hbs.
Further details can be found here: https://getpublii.com/dev/how-to-create-custom-templates/

In the file, locate the {{#each post}} helper and replace it with:

{{#each @website.contentStructure.tags}}

Then, add the following code:

{{#checkIf this.name '==' 'TAGNAME'}}
    {{/each}}
    {{#if this.postsNumber}}
    {{#each this.posts}}

Replace TAGNAME with the desired tag (the name, not the slug). In this case, use 'Mallorca' instead of 'mallorca.'

Finally, locate the closing {{/each}} helper (from the original {{#each post}}) and insert the following code just before it:

{{/each}}
{{/if}}
{{/checkIf}}

Additional information on this topic can be found here:
https://getpublii.com/dev/how-to-display-posts-connected-with-a-specific-tag-name/

The post-newname.hbs file is now available as a post template. See the screenshot below. Create a new post and select this template:

For the masonry effect, ensure that in the footer.hbs, the script functionality is extended to the new template. In this case, add 'post-mallorca' as follows:

{{#is "index,tag,tags,author,post,post-mallorca"}}
	<script defer src="{{js "isotope.pkgd.min.js"}}"></script> 

Put the footer.hbs in the override folder in a sub folder partials.

If you don't want the posts in this collection to appear on the homepage, mark them in Publii as "Exclude from homepage."