# Building Custom UI Gradio Apps with HTML/CSS/JS

This guide shows how to create Gradio apps with fully custom HTML/CSS/JS interfaces that call Python functions. Perfect for deploying on Hugging Face Spaces while maintaining complete control over your UI.

## Overview

Gradio 6.0+ allows you to:
- Replace the native Gradio UI with custom HTML/CSS/JS
- Call Python functions from JavaScript using `trigger()`
- Use template syntax to display Python results dynamically

## Basic Structure

```python
import gradio as gr

def my_python_function(data):
    """Your Python logic here"""
    result = data.get("input") * 2
    return {"output": result}

HTML = """
<div id="app">
  <input type="text" id="my-input">
  <button id="submit-btn">Submit</button>
  <div id="result">${value.output || 'No result yet'}</div>
</div>
"""

CSS = """
<style>
  #app { padding: 20px; }
  /* your styles */
</style>
"""

JS = """
(function() {
  function $(sel) { return element.querySelector(sel); }

  $('#submit-btn').addEventListener('click', function() {
    // Set the data to send to Python
    props.value = { input: $('#my-input').value };
    // Call the Python function
    trigger('submit');
  });
})();
"""

with gr.Blocks() as demo:
    component = gr.HTML(
        value={},
        html_template=CSS + HTML,
        js_on_load=JS,
        container=False
    )
    component.submit(my_python_function, inputs=component, outputs=component)

demo.launch()
```

## Key Concepts

### 1. The `gr.HTML` Component

In Gradio 6.0+, `gr.HTML` has special parameters:

| Parameter | Description |
|-----------|-------------|
| `value` | Initial data (use `{}` for empty object) |
| `html_template` | Your HTML with template syntax `${value.xxx}` |
| `js_on_load` | JavaScript that runs when component loads |
| `container` | Set to `False` to remove Gradio's wrapper |

### 2. JavaScript API

Inside `js_on_load`, you have access to:

- **`element`** - The DOM element of your component
- **`props`** - Object containing `value` and other properties
- **`trigger(event)`** - Function to trigger Gradio events

```javascript
// Access DOM elements within your component
element.querySelector('#my-button')

// Update the value (this is what Python receives)
props.value = { key: "value" }

// Call Python function
trigger('submit')  // triggers .submit() handler
trigger('click')   // triggers .click() handler
```

### 3. Template Syntax

Use JavaScript template literals in `html_template`:

```html
<!-- Simple value -->
<div>${value.result}</div>

<!-- With fallback -->
<div>${value.result || 'Default text'}</div>

<!-- Conditional -->
<div>${value.result !== undefined ? value.result : '—'}</div>
```

### 4. Python Handler

The Python function receives `props.value` as its input:

```python
def handler(data):
    # data is whatever you set in props.value
    num1 = data.get("num1", 0)
    num2 = data.get("num2", 0)

    # Return a dict - this becomes the new props.value
    return {"result": num1 + num2}
```

### 5. Wiring It Up

Connect the JavaScript trigger to Python:

```python
component.submit(handler, inputs=component, outputs=component)
```

When JS calls `trigger('submit')`, Gradio:
1. Sends `props.value` to `handler()`
2. Takes the return value
3. Updates `props.value` with it
4. Re-renders the HTML template

## Hiding Gradio's Native UI

To make your app look completely custom, add CSS to hide Gradio's chrome:

```css
/* Hide footer */
footer { display: none !important; }

/* Reset containers */
.gradio-container, [class*="gradio-container"] {
  max-width: 100% !important;
  width: 100% !important;
  padding: 0 !important;
  margin: 0 !important;
}

/* Remove padding from main */
main.fillable, main[class*="fillable"], .app {
  padding: 0 !important;
  margin: 0 !important;
}

/* Clean up blocks */
.block, .block.padded {
  padding: 0 !important;
  border: none !important;
  background: transparent !important;
}

/* Reset svelte components */
[class*="svelte"] {
  padding: 0 !important;
  margin: 0 !important;
}
```

## Hugging Face Spaces: Avoiding Iframe Issues

HF Spaces use an iframe resizer. Using `min-height: 100vh` causes infinite scroll. Instead:

```css
html, body {
  height: 100%;
  overflow: hidden;  /* NOT auto */
}

#app {
  height: 100%;
  /* Use 100% not 100vh - viewport units cause iframe resizer loops */
}

/* Prevent iframe resizer issues */
html, body, #root, .gradio-container, .contain, gradio-app {
  height: 100% !important;
  overflow: hidden !important;
  min-height: 0 !important;  /* Important! Removes default min-height */
}

.contain, .main, gradio-app {
  min-height: 0 !important;
  max-height: 100% !important;
}
```

**Key rule:** Never use `vh` units (`100vh`, `min-height: 100vh`). Always use `100%` which is relative to the parent, not the viewport.

## Complete Example: Math Calculator

```python
import gradio as gr

def add_numbers(data):
    num1 = float(data.get("num1", 0))
    num2 = float(data.get("num2", 0))
    result = num1 + num2
    return {"result": result, "expression": f"{num1} + {num2} = {result}"}

HTML = """
<div id="app">
  <h1>Calculator</h1>
  <input type="number" id="num1" value="0">
  <span>+</span>
  <input type="number" id="num2" value="0">
  <button id="calc-btn">Calculate</button>
  <div id="result">${value.result !== undefined ? value.result : '—'}</div>
  <div id="expr">${value.expression || ''}</div>
</div>
"""

CSS = """
<style>
* { margin: 0; padding: 0; box-sizing: border-box; }
html, body { height: 100%; overflow: hidden; }
#app {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 100%;
  background: #1a1a2e;
  color: white;
  gap: 1rem;
}
input { padding: 0.5rem; font-size: 1.2rem; width: 100px; }
button { padding: 0.5rem 1rem; cursor: pointer; }
#result { font-size: 2rem; color: #00d2ff; }

/* Hide Gradio UI */
footer { display: none !important; }
.gradio-container { max-width: 100% !important; padding: 0 !important; }
</style>
"""

JS = """
(function() {
  function $(sel) { return element.querySelector(sel); }

  $('#calc-btn').addEventListener('click', function() {
    props.value = {
      num1: parseFloat($('#num1').value) || 0,
      num2: parseFloat($('#num2').value) || 0
    };
    trigger('submit');
  });

  // Optional: Enter key support
  ['#num1', '#num2'].forEach(function(sel) {
    $(sel).addEventListener('keypress', function(e) {
      if (e.key === 'Enter') $('#calc-btn').click();
    });
  });
})();
"""

with gr.Blocks(fill_height=True, title="Calculator") as demo:
    calc = gr.HTML(
        value={},
        html_template=CSS + HTML,
        js_on_load=JS,
        container=False
    )
    calc.submit(add_numbers, inputs=calc, outputs=calc)

if __name__ == "__main__":
    demo.launch()
```

## Tips & Gotchas

### DO:
- Use `element.querySelector()` instead of `document.querySelector()` (scoped to component)
- Use IIFE `(function() { ... })();` to avoid global scope pollution
- Return a dict from Python - it becomes `props.value`
- Use `${value.xxx}` template syntax for dynamic content
- Set `container=False` on `gr.HTML` for cleaner output

### DON'T:
- Use inline `onclick="fn()"` handlers (they won't work due to scoping)
- Use `min-height: 100vh` (breaks HF iframe resizer)
- Forget to connect `.submit()` handler
- Use `document.getElementById()` (use `element.querySelector('#id')`)

### Common Patterns

**Status Updates:**
```javascript
function updateStatus(msg) {
  element.querySelector('#status').textContent = msg;
}
$('#btn').addEventListener('click', function() {
  updateStatus('Processing...');
  trigger('submit');
});
```

**Multiple Event Types:**
```python
component.submit(handle_submit, inputs=component, outputs=component)
component.click(handle_click, inputs=component, outputs=component)
```

**Error Handling:**
```python
def handler(data):
    try:
        result = do_something(data)
        return {"success": True, "result": result}
    except Exception as e:
        return {"success": False, "error": str(e)}
```

## File Structure for HF Spaces

```
your-space/
├── app.py           # Main Gradio app
├── requirements.txt # Just: gradio>=6.0.0
└── README.md        # HF Space metadata
```

`README.md` for HF Spaces:
```yaml
---
title: Your App Name
emoji: 🚀
colorFrom: blue
colorTo: purple
sdk: gradio
sdk_version: 6.0.1
app_file: app.py
pinned: false
---
```

## Resources

- [Gradio HTML Component Docs](https://www.gradio.app/docs/gradio/html)
- [Custom HTML Components Guide](https://www.gradio.app/guides/custom_HTML_components/)
- [Hugging Face Spaces](https://huggingface.co/spaces)

---

Built with Gradio 6.0+ | Works on Hugging Face Spaces with ZeroGPU