WhizKins

WPSimpleInsert is a plugin to insert custom code in your blog posts. It includes options to either add the code above 
or below post content.
Features:
<ul>
<li>Add custom HTML/PHP/JavaScript to your posts.</li>
<li>Ability to add custom code above or below post content.</li>
<li>Options to hide the code from certain pages like the home page and category and archives pages.</li>
</ul>

WP Simple Insert

A plugin to insert custom HTML or PHP code in your posts. Includes options to insert before or after post content.

YAHMAN

ZIPANG

When you enter the HTML code directly, the layout changes depending on the p and br tags that WordPress automatically inserts.
<pre><code>remove_filter( 'the_content', 'wpautop' );
</code></pre>
It can be solved by adding the above code. 
However, this will disable all auto-inserted p and br tags. 
It is not user-friendly.
This plugin can be partially disabled.
Just enter the part you want to disable in the shortcode.
<pre><code>[jo]your HTML code[/jo]
</code></pre>
The author of this plugin loves text editors.

Just Output

WordPress automatically inserts p and br tags, but disables them in shortcodes.

WP Simple Insert vs Just Output