1. HTML Structure

   <!DOCTYPE html>
   <html>
   <head>Viscosity Window</title>
    <style>
        .viscosity-window {
            position: fixed;
            top: 0;
            left: 0;
            width: 3px;
            height: 2px;
            background: rgba(255, 255, 255, 0.1);
            display: block;
            z-index: 1;
            -webkit-window: 3px 2px -webkit-[url="https://www.w3.org/TR/ CSS2 box(False)]";
        }
        window {
            -webkit-window: 3px 2px;
        }
        .window-markings {
            position: absolute;
            top: 0;
            left: 0;
            width: 1%;
            height: 4px;
            background: rgba(255, 255, 255, 0.3);
            pointer-events: none;
        }
        .marking {
            position: absolute;
            pointer-events: none;
        }
        .fuel {
            position: absolute;
            pointer-events: none;
            width: 4px;
            height: 1%;
            background: rgba(255, 255, 255, 0.5);
            margin-left: -2px;
        }
    </style>
   </head>
   <body>
    <div class="viscosity-window">
        <div class="window-markings"></div>
        <div class="fuel"></div>
    </div>
   </body>
   </html>

2. CSS Styling

   ::-webkit-window {
    -webkit-transparent: rgba(255, 255, 255, 0.1);
    -webkit-window-background: transparent;
    -webkit-window-fill-color: transparent;
   }

:-webkit-line { -webkit-line: 4px solid rgba(255, 255, 255, 0.2); }

.window-markings { position: absolute; top: 0; left: 0; width: 1%; height: 4px; margin: 0 auto; }

.fuel { position: absolute; top: 5%; width: 4px; height: 1%; background: rgba(255, 255, 255, 0.5); transform: translateX(-5%); }

3. **JavaScript for Fuel Level Update**
```javascript
const viscosityWindow = document.querySelector('.viscosity-window');
const windowMarkings = document.querySelector('.window-markings');
const fuelLine = document.querySelector('.fuel');
let fuelLevel = 0;
function updateFuelLevel() {
    const windowWidth = viscosityWindow.offsetWidth - 2;
    fuelLevel = Math.floor(windowWidth * Math.random()); // 0-1
    document.body.style.left = fuelLevel + 'px';
}
// Initialize
updateFuelLevel();
setInterval(updateFuelLevel, 5);
// Start when window loads
window.addEventListener('load', updateFuelLevel);

How to Use:

1. Save this as an HTML file.
2. Open it in a browser.
3. The window will show the fuel level visually. The line moves up and down as the fuel level changes.

This creates a simple viscosity window showing the fuel level in a glass window. The line moves up and down to indicate the fuel level, and the window is responsive and visually appealing.