Janis Elsts

Lets you disable automatic formatting like smart quotes and automatic paragraph creation, and use raw HTML/JS/CSS code in your posts without WordPress messing it up.
Features
With this plugin, you can wrap any part of your post in [raw]…[/raw] tags to prevent WordPress from converting newlines to HTML paragraphs, replacing apostrophes with typographic quotes and so on. This is very useful if you need to add a CSS block or JavaScript to your post.
RawHTML will also add new checkboxes to the “Edit Post” screen that let you disable certain WP filters on a per-post basis. This way you can:
<ul>
<li>Disable wptexturize (the function that creates smart quotes and other typographic characters).</li>
<li>Disable automatic paragraph creation.</li>
<li>Disable image smilies. </li>
<li>Disable convert_chars (the function that converts ampersands to HTML entities and “fixes” some Unicode characters).</li>
</ul>
The free version only supports editing posts in the Text tab (called “HTML” in older WordPress versions). <a href="http://rawhtmlpro.com/?utm_source=wordpress.org&utm_medium=readme_link&utm_campaign=RawHTML%20free" rel="nofollow ugc">Get the Pro version</a> if you want to be able to switch between Text and the Visual editor without WordPress messing up your content.
Compatibility
This plugin doesn’t fully support the Gutenberg editor. As of WordPress 5.0, some Raw HTML features will only work if you use the <a href="https://wordpress.org/plugins/classic-editor/" rel="ugc">Classic Editor</a> plugin.
Usage
To prevent a part of your post or page from being filtered by WordPress, switch to the Text/HTML editor and wrap it in <code>[raw]...[/raw]</code> or <code>...</code> tags. These two versions work exactly the same, except that the latter won’t be visible to your visitors even if you deactivate Raw HTML.
Example :
<pre><code>[raw]
This 

is 

a "test"!
[/raw]
</code></pre>
In this case, the tags will prevent WordPress from inserting paragraph breaks between “This”, “is” and “a “test””, as well as ensure that the double quotes arround “test” are not converted to typographic (curly) quotes.
To avoid problems, only edit posts that contain your custom code in Text/HTML mode. If you’d like to be able to also use the Visual editor, <a href="http://rawhtmlpro.com/?utm_source=wordpress.org&utm_medium=readme_link&utm_campaign=RawHTML%20free" rel="nofollow ugc">get the Pro version</a>. It will make the code betwen [raw] tags appear as a read-only placeholder when viewed in Visual mode, ensuring WordPress doesn’t change it.
Combining shortcodes
By default, shortcodes that are inside [raw] tags will not work. They will just show up as plain text. To enable shortcodes, add the <code>shortcodes=1</code> attribute to the tag:
<pre><code>[raw shortcodes=1]This [shortcode] will be run.[/raw]

[raw]This [shortcode] won't work.[/raw]
</code></pre>
Preserving <code>[raw]</code> code in excerpts
By default, the plugin will automatically remove any code that’s inside <code>[raw]...[/raw]</code> tags from post excerpts. You can prevent that by adding the following line to <code>wp-config.php</code>:
<pre><code>define('RAW_HTML_KEEP_RAW_IN_EXCERPTS', true);
</code></pre>
This will ensure that the plugin doesn’t strip <code>[raw]</code> blocks from automatically generated excerpts.
Notes
Some features of Raw HTML will only work for users who have the “unfiltered_html” capability. In a normal WordPress install that includes the Editor and Administrator roles. In a Multisite install, only the Super Admin has this capability by default.

Raw HTML

Simply works

Lets you use raw HTML or any other code in your posts. You can also disable smart quotes and other automatic formatting on a per-post basis.

Tammy Hart

Aaron Forgue

The Custom Class Selector plugin allows users to style their post content using 
custom classes made available by the active theme. Theme developers can make 
custom style classes available within the visual editor by adding a simple 
function to the functions.php file included with their theme.

Custom Class Selector

Allows users to style their post content using custom classes made available by the active theme.

Raw HTML vs Custom Class Selector