In HTML, the `<sub>` and `<sup>` tags are used to create subscript and superscript text. Subscripts are commonly found in chemical formulas and mathematical expressions, while superscripts are often used for exponentiation and footnote markers. These inline elements do not cause line breaks and can be styled with CSS, including font size, color, and vertical alignment. Typical use cases include chemical molecular formulas, mathematical equations, ordinal notations, and footnote markers. They can be nested or combined with other elements, such as links. In complex mathematical expressions, multi-level nesting may be required. Screen readers recognize these tags, and it is recommended to add `aria-label` for complex expressions. All modern browsers support these tags, making them lighter than MathML for simple scenarios. When using them, ensure semantic correctness, adjust font size in responsive designs, and note that some fonts may display poorly. Special requirements can be simulated with CSS.
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In HTML, the `strong` and `em` tags are semantic tags specifically designed for text emphasis. `strong` indicates the importance of content, while `em` conveys emphasis in tone. By default, `strong` is displayed in bold and `em` in italics, but their core purpose lies in semantics rather than styling. They can be nested to express complex semantic hierarchies. Unlike the purely presentational `b` and `i` tags, they carry clear semantic meaning, making them particularly useful in scenarios like form validation, technical documentation, and more. Screen readers adjust their intonation based on these tags, enhancing accessibility. CSS can be used to customize their display styles, including responsive design and print optimization. In dynamic content and internationalization contexts, these tags should be used appropriately to improve interactive feedback and adapt to different language environments.
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