A style is not applied to all the selected text in Word

Consider the following scenario. In a Microsoft Word document, you select several lines of text on which various kinds of formatting have been applied. Then, you apply a style to the selected text. In this scenario, you may notice that the style is not applied to all the selected text. You may have to select each line of text separately and then apply the style.

This behavior occurs because Word uses a specific rule to determine whether to apply a style to selected text. According to this rule, Word applies a style depending on the percentage of the selected text that already has formatting applied. For example, if you already applied formatting to less than 50 percent of the selected text, this formatting is retained when you apply a style. If the selected text includes multiple paragraphs, Word first calculates the percentage of text that is formatted in the first paragraph. Then, it examines the paragraphs in the same range. If the formatting that is applied to the text in the paragraphs that follow the first paragraph differs from most of the formatting in the first paragraph, Word does not apply the style to the following paragraphs. Therefore, the formatting is retained in these paragraphs.

Word is designed to use this rule for performance reasons. For example, you have a document that has multiple pages that have multiple paragraphs. If you select all the text in the document and then apply the Normal style, Word has to retrieve and calculate the formatting that is applied to most of the text in every paragraph in the document.

Steps to reproduce the behavior

1. Open a Word document.
2. Type the following text:
   This line will test how styles and formatting work in Word.
3. Select all the text, and then apply italic formatting.
4. Select all the text, and then apply a style such as Heading 1.
   
   You notice that italic formatting is not retained.
5. Select all the text, apply the Normal style, and then remove the italic formatting.
6. Select "work in Word" in the text, and then apply the italic formatting.
7. Select all the text, and then apply the Heading 1 style.

You notice that the italic formatting is retained.