Редакторе WordPress поддерживает определенный набор HTML тэгов (т.е. не все тэги из спецификации HTML работаю). Так же в редакторе есть кнопки для вставки тэгов их написание несколько отличается от самого тэга

Выдержка из документации Code

HTML Tags

WordPress.com allows the following HTML code in your posts, pages, and widgets:

address, a, abbr, acronym, area, b, big, blockquote, br, caption, cite, class, code, col, del, dd, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, img, ins, kbd, li, map, ol, p, pre, q, s, span, strike, strong, sub, sup, table, tbody, td, tfoot, tr, tt, ul, var

Check out W3 Schools for more information about what each of these HTML codes are used for.

If you are familiar with HTML, you’ll notice that codes such as embed, frame, iframe, form, input, object, textarea and others are missing from the above list. Those codes are not allowed on WordPress.com for security reasons.

Тэги и примеры

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b-quote — <blockquote>
The <blockquote> tag defines a long quotation.

A browser inserts white space before and after a blockquote element. It also insert margins for the blockquote element.

Цитирование. Браузер вставляет пустое место до и после абзаца и делает отступ.

Here is a long quotation here is a long quotation here is a long quotation here is a long quotation here is a long quotation here is a long quotation here is a long quotation here is a long quotation here is a long quotation.

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del — <del>
The <del> tag defines text that has been deleted from a document.Удаленный текст (текст будет зачеркнут).

My favorite color is blue!

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ins — <ins>
The <ins> tag defines text that has been inserted into a document.Вставленный текст (можно выделять цветом).

My favorite color is red!

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ul — <ul>
The <ul> tag defines an unordered list (a bulleted list).

• Coffee
• Tea
• Milk

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ol — <ol>
The <ol> tag is used to create an ordered list. The list can be numerical or alphabetical.

Упорядоченный список.

1. Coffee
2. Tea
3. Milk

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li -<li>

The <li> tag defines a list item. The <li> tag is used in both ordered (<ol>) and unordered (<ul>) lists.

Элемент списка.

1. Coffee
2. Tea
3. Milk

• Coffee
• Tea
• Milk

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code — <code> и другие
The <em>,<strong>,<code>,<kbd>,<var>, and <cite> tags are all phrase tags.
They are not deprecated, but it is possible to achieve richer effect with CSS.
Блок исходных кодов.

<em> Emphasized text
<strong> Strong text
<code> Computer code text
<kbd> Keyboard text
<var> Variable
<cite> Citation

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more — <!—more—>

Вставка разрыва сообщения (кат). Т.е. будет показан кусок сообщения выше этого тэга и ссылка «Читать далее» на остаток сообщения.