The viewport tag contains important information, telling the web browser how to scale your webpage to make it responsive. A responsive page scales to look appropriate on different-size devices (i.e. computer, smartphone, smart TV).
<span class="token doctype"><span class="token punctuation"><!</span><span class="token doctype-tag">DOCTYPE</span> <span class="token name">html</span><span class="token punctuation">></span></span>
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<span class="token tag"><span class="token punctuation"><</span>meta <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">'</span>viewport<span class="token punctuation">'</span></span> <span class="token attr-name">content</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">'</span>width=device-width, initial-scale=1.0<span class="token punctuation">'</span></span><span class="token punctuation">></span></span>
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width=device-width sets the width of the page to the width of the device and initial-scale=1.0 sets the page zoom.