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Big bold text doesn't count as a real heading. A heading in InDesign isn't really a heading until it has been mapped to a heading tag.
Step-by-
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Step Guide to
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Creating Paragraph Styles and Mapping to Heading Tags in Adobe InDesign
In the top bar, click the Type Tab. In the dropdown menu, click Paragraph Styles.
Create a new blank style by clicking the New Style icon in the bottom right corner. Double-click on the new style to open the Style Editor.
In the Style Editor, go to the General Category, type in a Style Name.
Go to the Basic Character Formats Category. Modify the styles appearance in what ever way suits your needs. You can change the fonts, size, positions, margins, etc).
Go to the Export Tagging category. Click the EPUB and HTML tag dropdown menu, and choose your tag. Choose the same tag in the PDF Tag dropdown menu. Click OK to exit the box.
To apply the heading to the text, place the cursor on the text in the document. Select the heading level from the Paragraph Styles pane. The text will change to the new style.
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