Read about Nyquist and Waveform Shape
The maximum frequency that a digital oscillator can produce is defined as the Nyquist Rate which is half the sample rate. The current sample rate for your output device is : Undefined samples per second. If you change the output device, refresh this page as the sample rate may be different. The maximum frequency this browser is able to generate may be lower, this will be reflected in the maximum permissible setting of the frequency input.
The audio may be filtered by your sound card, the output level may be lower than expected at extremes of low or high frequencies. If you require accurate frequencies, levels and waveforms, a computer or phone is no substitute for a specialist signal generator.
The shape of the output waveform from this generator is dictated by the ratio of frequency to sample rate. Although the browser's oscillator offers various waveform shapes, this only holds true for lower frequencies. For example, at the common 48,000 samples per second, a tone of 400Hz will be made up of 120 samples per cycle, whereas 2.5kHz will have just 19. At the Nyquist frequency there are just two samples, one positive and one negative going.
The sound card may also distort the waveform shape by filtering or other effects such as bass or treble boosts.
Have fun, but be aware of the limitations of your device.