Tumblr's rich-text editor doesn't support most HTML Web programming code and will either strip it out or publish it as text. However, Tumblr enables you to bold text in this default editor, similar to how you'd bold text in a word processor. You can also enable the optional plain-text editor, which supports basic HTML code.
Applying the Bold Effect
Using the rich-text editor, clicking the "B" at the top of the editor applies the bold effect to the selected text within the editor window. If you're more comfortable with manual coding, click the "Settings" gear icon and then choose the "Plain Text/HTML" option in the "Edit Posts Using" section. This editor recognizes that text enclosed by the "" and "" tags should be bold, but the bold effect is applied only on the preview or publication page. The "B" button is still available in the plain-text editor, but it simply adds the HTML tags for you without displaying the bold effect in the editor window.
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