There are several design principles and guidelines covered in the optional subject that are essential for creating visually appealing and functional designs. These principles and guidelines help designers create effective and user-friendly designs. Here are some of the key principles and guidelines:

1. Balance:
- Achieving visual balance is crucial in design.
- Balance can be symmetrical, asymmetrical, or radial.
- It helps create a sense of stability and harmony in the design.

2. Contrast:
- Contrast refers to the difference between elements in a design.
- It helps create visual interest and hierarchy.
- Contrast can be achieved through color, size, shape, texture, etc.

3. Emphasis:
- Emphasis is used to draw attention to a specific element or area in a design.
- It helps create focal points and guides the viewer's eye.
- Emphasis can be achieved through color, size, contrast, or positioning.

4. Unity:
- Unity refers to the cohesive and harmonious relationship between elements in a design.
- It helps create a sense of completeness and coherence.
- Unity can be achieved through consistent use of colors, fonts, styles, etc.

5. Proportion:
- Proportion deals with the size and scale of elements in relation to each other and the overall design.
- It helps create a balanced and visually pleasing composition.
- Proportion can be used to create emphasis, hierarchy, and depth.

6. Typography:
- Typography refers to the selection and arrangement of fonts in a design.
- It plays a crucial role in readability and aesthetics.
- Guidelines for typography include using appropriate fonts, sizes, spacing, and hierarchy.

7. Color Theory:
- Color theory explores the principles and guidelines for using color effectively in design.
- It includes understanding the color wheel, color harmonies, and the psychological effects of color.
- Color theory helps create visual impact, convey messages, and evoke emotions.

8. User Experience (UX) Design:
- User experience design focuses on creating designs that are user-friendly and intuitive.
- It involves understanding user behavior, conducting usability testing, and incorporating feedback.
- UX design principles and guidelines aim to enhance the usability and overall experience of the design.

By understanding and applying these design principles and guidelines, designers can create visually appealing, functional, and user-friendly designs. These principles serve as a foundation for effective design and help in creating designs that communicate messages effectively and engage with the audience.