The data, enclosed within the <script type="application/ld+json"> ... </script> tags as shown in the examples below may be placed in either the <HEAD> or <BODY> region of the page. Either way, it won’t affect how your document appears in user’s web browsers.

The bible: Google Schemas Also very important: JSON-LD vs microdata Google Article Rich Snippets Guide Updated for AMP Structured Data on the Web

Steal Our JSON-LD | JSON-LD Schema Markup Examples

Full Hierarchy - schema.org

The best explained gude with a perfect example needed for us: Microdata, JSON-LD & Schema: Rich Snippets Guide

JSON-LD in Real Life: Understanding and Implementing Structured Data | Distilled

The rules for aggregate review

Review snippet guidelines

Don’t add review stars to all pages at once, but add them gradually over time
Don’t try to place them on your homepage

Wordpress Plugins

Is there any special support for AMP pages or not? It has to have, or not?

JSON-LD for Article is discontinued but it was the simplest solution without any settings. There is more expanded fork wp-jsonld and it plays nicely with YASR review plugin.

Thoroughly reviewed:

WPSSO is huge behemoth, but it actually is not that bad if you also need social markup or even Social Sharing Buttons. There is a Pro version, but lots of extensions are free like:
- JSON-LD Markup should be included out of the box
- Social Buttons
- Strip Schema Microdata to remove outdated theme microdata.
Only when you dig deeper, you notice this message: The Free / Basic version of WPSSO JSON does not include support for the Schema type https://schema.org/Product. So no, it doesn’t support Product Schema Type in free version.
WP SEO Structured Data Schema is very straightforward and supports AggregateRating out of the box. Problem is that for now, there is no way to use it on custom post types. There is also no way to set any default values. Simple, small but too limited.
Schema / home / extensions Review. I like this plugin as it is simple, fast and straightforward. It doesn’t support AggregateRating (only reviewRating via free extension) and I have found no way to add Product schema type - schema types will be added in the future.
All In One Schema.org Rich Snippets supports is by far the most popular one, but it is used for presenting manually entered reviews on front-end, by using metabox in every post type. Supports AggregateRating, but it doesn’t support JSON-LD. It is not really what I need.

Just mentioned:

Markup (JSON-LD) structured in schema.org is simple, but there is no AggregateRating support
Google SEO Pressor for Rich snippets is not using JSON-LD, but old microformat.
WP Review Schema is not maintained anymore.

Use the right HTML5 tags for better HTML structure

HTML 5 Tags

Deciding on markup in tables with logos. In paragraph: Example 14.3 of this official resource draft: HTML5: Techniques for providing useful text alternatives, it is clearly stated to specify empty alt attribute.
Use simple span just to explain something, like: <span title="Explanation">something</span>.
If it is abbreviation, a shortened form of a word or phrase, use <abbr title="Unknown Flying Object">UFO</abbr>
section is generic semantic element, just like div when you need styling.